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(Frontispiece.) 


THE  MECHANISM 

OF 

MENDELIAN  HEREDITY 


BY 


T.  H.  MORGAN 

PROFESSOR   OF   EXPERIMENTAL   ZOOLOGY 
COLUMBIA    UNIVERSITY 


A.  H.  STURTEVANT 

CUTTING   FELLOW,   COLUMBIA   UNIVERSITY 

H.  J.  MULLER 

ASSISTANT  IN   ZOOLOGY,   COLUMBIA   UNIVERSITY 

C.  B.  BRIDGES 

FELLOW   IN   ZOOLOGY,   COLUMBIA   UNIVERSITY 


NEW  YORK 
HENRY  HOLT  AND  COMPANY 


COPYRIGHT,  1915 
BY 

HENRY  HOLT  AND  COMPANY 
2 


THK    MA.PLE     PRESS     YORK.    PA. 


Go 

EDMUND  BEECHER  WILSON 


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PREFACE 

From  ancient  times  heredity  has  been  looked  upon 
as  one  of  the  central  problems  of  biological  philoso- 
phy. It  is  true  that  this  interest  was  largely  specu- 
lative rather  than  empirical.  But  since  Mendel's 
discovery  of  the  fundamental  law  of  heredity  in 
1865,  or  rather  since  its  re-discovery  in  1900,  a  curious 
situation  has  begun  to  develop.  The  students  of 
heredity  calling  themselves  geneticists  have  begun 
to  draw  away  from  the  traditional  fields  of  zoology 
and  botany,  and  have  concentrated  their  attention 
on  the  study  of  Mendel's  principles  and  their  later 
developments.  The  results  of  these  investigators 
appear  largely  in  special  journals.  Their  terminology 
is  often  regarded  by  other  zoologists  as  something 
barbarous, — outside  the  ordinary  routine  of  their  pro- 
fession. The  tendency  is  to  regard  genetics  as  a  sub- 
ject for  specialists  instead  of  an  all-important  theme  of 
zoology  and  botany.  No  doubt  this  is  but  a  passing 
phase;  for  biologists  can  little  afford  to  hand  over  to 
a  special  group  of  investigators  a  part  of  their  field 
that  is  and  always  will  be  of  vital  import.  It  would 
be  as  unfortunate  for  all  biologists  to  remain  ignorant 
of  the  modern  advances  in  the  study  of  heredity  as 
it  would  be  for  the  geneticists  to  remain  unconcerned 

vii 


viii  PREFACE 

as  to  the  value  for  their  own  work  of  many  special 
fields  of  biological  inquiry.  What  is  fundamental 
in  zoology  and  botany  is  not  so  extensive,  or  so  in- 
trinsically difficult,  that  a  man  equipped  for  his 
profession  should  not  be  able  to  compass  it. 

In  the  following  pages  we  have  attempted  to  sepa- 
rate those  questions  that  seem  to  us  significant 
from  that  which  is  special  or  merely  technical.  We 
have,  of  course,  put  our  own  interpretation  on  the 
facts,  and  while  this  may  not  be  agreed  to  on  all  sides, 
yet  we  believe  that  in  what  is  essential  we  have  not 
departed  from  the  point  of  view  that  is  held  by  many 
of  our  co-workers  at  the  present  time.  Exception 
may  perhaps  be  taken  to  the  emphasis  we  have  laid 
on  the  chromosomes  as  the  material  basis  of  in- 
heritance. Whether  we  are  right  here,  the  future— 
probably  a  very  near  future — will  decide.  But  it 
should  not  pass  unnoticed  that  even  if  the  chromo- 
some theory  be  denied,  there  is  no  result  dealt  with 
in  the  following  pages  that  may  not  be  treated  inde- 
pendently of  the  chromosomes;  for,  we  have  made 
no  assumption  concerning  heredity  that  cannot  also 
be  made  abstractly  without  the  chromosomes  as 
bearers  of  the  postulated  hereditary  factors.  Why 
then,  we  are  often  asked,  do  you  drag  in  the  chro- 
mosomes? Our  answer  is  that  since  the  chromo- 
somes furnish  exactly  the  kind  of  mechanism  that 
the  Mendelian  laws  call  for;  and  since  there  is  an 
ever-increasing  body  of  information  that  points 
clearly  to  the  chromosomes  as  the  bearers  of  the 


PREFACE  IX 

Mendelian  factors,  it  would  be  folly  to  close  one's 
eyes  to  so  patent  a  relation.  Moreover,  as  biologists, 
we  are  interested  in  heredity  not  primarily  as  a  mathe- 
matical formulation  but  rather  as  a  problem  concern- 
ing the  cell,  the  egg,  and  the  sperm. 

T.  H.  M. 


CONTENTS 

CHAPTER  I 
MENDELIAN  SEGREGATION  AND  THE  CHROMOSOMES 

PAGE 

Introduction.     The  Groups  of  Linked   Factors  and   the  Chromo- 
somes      1 

The  Inheritance  of  One  Pair  of  Factors 8 

The  Inheritance  of  Two  or  more  Pairs  of  Factors 20 

CHAPTER  II 
TYPES  OF  MENDELIAN  HEREDITY 

Dominance  and  Recessiveness 27 

Manifold  Effects  of  Single  Factors 32 

Similar  Effects  Produced  by  Different  Factors 36 

Modification  of  the  Effects  of  Factors 38 

I.  By  Environmental  Influences 38 

II.  By  Developmental  Influences 42 

III.  By  the  Influence  of  Other  Factors 45 

IV.  Conclusion 46 

CHAPTER  III 
LINKAGE 

Examples  Illustrating  "Coupling" 48 

Examples  Illustrating  "Repulsion" 51 

Examples  of  Different  Frequencies  of  Crossing  Over 52 

The  Mechanism  of  Crossing  Over 59 

Double  Crossing  Over 62 

The  Principle  of  Interference 64 

The  Linear  Arrangement  of  Factors  shown  by  Linkage  Relations    .  64 

Linkage  in  Other  Animals  and  in  Plants 70 

The  Reduplication  Hypothesis 74 

xi 


Xll 


SEX  INHERITANCE 

.  PAGE 

The  Drosophila  or  XX-XY  Type 78 

The  Abraxas  or  WZ-ZZ  Type 83 

What  are  Sex  Factors?  90 


CHAPTER  V 

THE  CHROMOSOMES  AS  BEARERS  OF  HEREDITARY 
MATERIAL 

The  Evidence  from  Embryology 108 

The  Individuality  of  the  Chromosomes     118 

The  Chromosomes  during  the  Maturation  of  the  Germ  Cells    .    .    .  122 

Crossing  Over 131 

Cytoplasmic  Inheritance 135 

CHAPTER  VI 

THE  CORRESPONDENCE  BETWEEN  THE  DISTRIBUTION  OF 
THE  CHROMOSOMES  AND  OF  THE  GENETIC  FACTORS 

Parallelism  between  the  Distribution  of  Chromosomes  and  of  Factors  140 

1.  In  Cases  of  Normal  Distribution 140 

2.  In  Crosses  between  Species 141 

3.  In  Mutant  Races 146 

4.  Tetraploid  Races 147 

Identity    of    Distribution    of    the    X-chromosomes    and    of    Sex- 
linked  Factors 148 

1.  In  Ordinary  Crosses 148 

2.  In  Cases  of  Non-disjunction 149 

CHAPTER  VII 
MULTIPLE  ALLELOMORPHS 

Definition  of  Multiple  Allelomorphs 155 

Examples  of  Multiple  Allelomorphs 155 

The  Alternative  Interpretations  of  Identical  Loci  and  Complete 

Linkage .   157 


CONTENTS  Xlll 

CHAPTER  VIII 
MULTIPLE  FACTORS 

PAGE 

The  Meaning  of  the  term  "  Multiple  Factors  " 172 

Examples  of  Multiple  Factors 172 

Selection  and  Multiple  Factors 195 

CHAPTER  IX 
THE  FACTORIAL  HYPOTHESIS 

On  the  Relation  between  Factors  and  Characters 208 

1.  The  Organism-as-a- Whole  Objection 211 

2.  The  Invariability  of  the  Factor  and  the  Variability  in  the 

Character 212 

3.  So-called  Contamination  of  Allelomorphs 214 

4.  Fractionation 214 

5.  The  Presence  and  Absence  Hypothesis 216 

Weismann's  Prseformation  Hypothesis  and  the  Factorial  Theory    .  223 

APPENDIX 

Methods  of  Breeding  Drosophila 229 

Acknowledgments 235 

Bibliography 237 

INDEX  .  .  259 


THE   MECHANISM  OF 
MENDELIAN    HEREDITY 

CHAPTER  I 

MENDELIAN  SEGREGATION  AND  THE 
CHROMOSOMES 

Mendel's  law  was  announced  in  1865.  Its  funda- 
mental principle  is  very  simple.  The  units  con- 
tributed by  two  parents  -separate  in  the  germ  cells  of  the 
offspring  without  having  had  any  influence  on  each 
other.  For  example,  in  a  cross  between  yellow-seeded 
and  green-seeded  peas,  one  parent  contributes  to 
the  offspring  a  unit  for  yellow  and  the  other  parent 
contributes  a  unit  for  green.  These  units  separate 
in  the  ripening  of  the  germ  cells  of  the  offspring  so  that 
half  of  the  germ  cells  are  yellow  bearing  and  half  are 
green  bearing.  This  separation  occurs  both  in  the 
eggs  and  in  the  sperm. 

Mendel  did  not  know  of  any  mechanism  by  which 
such  a  process  could  take  place.  In  fact,  in  1865 
very  little  was  known  about  the  ripening  of  the  germ 
cells.  But  in  1900,  when  Mendel's  long-forgotten 
discovery  was  brought  to  light  once  more,  a  mechan- 
ism had  been  discovered  that  fulfils  exactly  the 
Mendelian  requirements  of  pairing  and  separation. 

The  sperm  of  every  species  of  animal  or  plant 

i 


2  MENDELIAN  SEGREGATION 

carries  a  definite  number  of  bodies  called  chromo- 
somes. The  egg  carries  the  same  number.  Conse- 
quently, when  the  sperm  unites  with  the  egg,  the 
fertilized  egg  will  contain  the  double  number  of 
chromosomes.  For  each  chromosome  contributed  by 
the  sperm  there  is  a  corresponding  chromosome  con- 
tributed by  the  egg,  i.e.,  there  are  two  chromosomes 
of  each  kind,  which  together  constitute  a  pair. 

When  the  egg  divides  (Fig.  1,  a-d),  every  chromo- 
some splits  into  two  chromosomes,  and  these  two 
daughter  chromosomes  then  move  apart,  going  to 
opposite  poles  of  the  dividing  cell  (Fig.  1,  c).  Thus 
each  daughter  cell  (Fig.  1,  d)  receives  one  of  the 
daughter  chromosomes  formed  from  each  original 
chromosome.  The  same  process  occurs  in  all  cell 
divisions,  so  that  all  the  cells  of  the  animal  or  plant 
come  to  contain  the  double  set  of  chromosomes. 

The  germ  cells  also  have  at  first  the  double  set  of 
chromosomes, but  when  they  are  ready  to  go  through 
the  last  stages  of  their  transformation  into  sperm 
or  eggs  the  chromosomes  unite  in  pairs  (Fig.  1,  e). 
Then  follows  a  different  kind  of  division  (Fig.  1 ,  /) 
at  which  the  chromosomes  do  not  split  but  the 
members  of  each  pair  of  chromosomes  separate  and 
each  member  goes  into  one  of  the  daughter  cells 
(Fig.  1 ,  g,  h) .  As  a  result  each  mature  germ  cell 
receives  one  or  the  other  member  of  every  pair  of 
chromosomes  and  the  number  is  reduced  to  half. 
Thus  the  behavior  of  the  chromosomes  parallels  the 
behavior  of  the  Mendelian  units,  for  in  the  germ  cells 
each  unit  derived  from  the  father  separates  from  the 


MENDELIAN    SEGREGATION 


corresponding  unit  derived  from  the  mother.  These 
units  will  henceforth  be  spoken  of  as  factors;  the 
two  factors  of  a  pair  are  called  allelomorphs  of  each 


a 


FIG.  1. — In  the  upper  line,  four  stages  in  the  division  of  the  egg  (or 
of  a  body  cell)  are  represented.  Every  chromosome  divides  when  the 
cell  divides.  In  the  lower  line  the  "reduction  division"  of  a  germ  cell, 
after  the  chromosomes  have  united  in  pairs,  is  represented.  The  mem- 
bers of  each  of  the  four  pairs  of  chromosomes  separate  from  each  other  at 
this  division. 

other.     Their  separation  in  the  germ  cells  is  called 
segregation. 

The  possibility  of  explaining  Mendelian  phenomena 


4  MENDELIAN    SEGREGATION 

by  means  of  the  manceuvers  of  the  chromosomes 
seems  to  have  occurred  to  more  than  one  per- 
son, but  Button  was  the  first  to  present  the  idea 
in  the  form  in  which  we  recognize  it  today.  More- 
over, he  not  only  called  attention  to  the  fact  above 
mentioned,  that  both  chromosomes  and  hereditary 
factors  undergo  segregation,  but  showed  that  the 
parallelism  between  their  methods  of  distribution 
goes  even  further  than  this.  Mendel  had  found  that 
when  the  inheritance  of  more  than  one  pair  of  factors 
is  followed,  the  different  pairs  of  factors  segregate 
independently  of  one  another.  Thus  in  a  cross  of  a 
pea  having  both  green  seeds  and  tall  stature  with  a 
pea  having  yellow  seeds  and  short  stature,  the  fact 
that  a  germ  cell  receives  a  particular  member  of  one 
pair  (e.g.,  yellow)  does  not  determine  which  member 
of  the  other  pair  it  receives;  it  is  as  likely  to  receive 
the  tall  as  the  short.  Sutton  pointed  out  that  in  the 
same  way  the  segregation  of  one  pair  of  chromosomes 
is  probably  independent  of  the  segregation  of  the 
other  pairs. 

It  was  obvious  from  the  beginning,  however,  that 
there  was  one  essential  requirement  of  the  chromo- 
some view,  namely,  that  all  the  factors  carried  by 
the  same  chromosome  should  tend  to  remain  together. 
Therefore,  since  the  number  of  inheritable  characters 
may  be  large  in  comparison  with  the  number  of  pairs 
of  chromosomes,  we  should  expect  actually  to  find 
not  only  the  independent  behavior  of  pairs,  but  also 
cases  in  which  characters  are  linked  together  in  groups 
in  their  inheritance.  Even  in  species  where  a  limited 


MENDELIAN    SEGREGATION  5 

number  of  Mendelian  units  are  known,  we  should  still 
expect  to  find  some  of  them  in  groups. 

In  1906  Bateson  and  Punnett  made  the  discovery 
of  linkage,  which  they  called  gametic  coupling.  They 
found  that  when  a  sweet  pea  with  factors  for  purple 
flowers  and  long  pollen  grains  was  crossed  to  a  pea 
with  factors  for  red  flowers  and  round  pollen  grains, 
the  twro  factors  that  came  from  the  same  parent 
tended  to  be  inherited  together.  Here  was  the  first 
case  that  gave  the  sort  of  result  that  was  to  be  ex- 
pected if  factors  were  in  chromosomes,  although  this 
relation  was  not  pointed  out  at  the  time.  In  the 
same  year,  however,  Lock  called  attention  to  the 
possible  relation  between  the  chromosome  hypothesis 
and  linkage. 

In  other  groups  a  few  cases  of  coupling  became 
known,  but  nowhere  had  the  evidence  been  sufficiently 
ample  or  sufficiently  studied  to  show  how  frequently 
coupling  occurs.  Since  1910,  however,  in  the  fruit 
fly,  Drosophila  ampelophila,  a  large  number  of  new 
characters  have  appeared  by  mutation,  and  so  rapidly 
does  the  animal  reproduce  that  in  a  relatively  short 
time  the  inheritance  of  more  than  a  hundred  char- 
acters has  been  studied.  It  became  evident  very  soon 
that  these  characters  are  inherited  in  groups.  There 
is  one  great  group  of  characters  that  are  sex  linked. 
There  are  two  other  groups  of  characters  slightly 
greater  in  number.  Finally  a  character  appeared 
that  did  not  belong  to  any  of  the  other  groups,  and  a 
year  later  still  another  character  appeared  that  was 
linked  to  the  last  one  but  was  independent  of  all  the 


8 


MENDELIAN    SEGREGATION 


other  groups.  Hence  there  are  four  groups  of  char- 
acters in  Drosophila.  A  partial  list  of  these  groups 
is  given  in  the  following  table : 


Group  I 

Group  II 

Group  III 

Abnormal 

Antlered 

Band 

Bar 

Apterous 

Beaded 

Bifid 

Arc 

Cream  III 

Bow 

Balloon 

Deformed 

Cherry 

Black 

Dwarf 

Chrome 

Blistered 

Ebony 

Cleft 

Comma 

Giant 

Club 

Confluent 

Kidney 

Depressed 

Cream  II 

Low  crossover 

Dotted 

Curved 

Maroon 

Eosin 

Dachs 

Peach 

Facet 

Extra  vein 

Pink 

Forked 

Fringed 

Rough 

Furrowed 

Jaunty 

Safranin 

Fused 

Limited 

Sepia 

Green 

Little  crossover 

Sooty 

Jaunty  I 

Morula 

Spineless 

Lemon 

Olive 

Spread 

Lethal  1 

Plexus 

Truncate  intens. 

Lethal  la 

Purple 

Trident 

Lethal  2 

Speck 

White  head 

Lethal  3 

Strap 

White  ocelli 

Lethal  3a 

Streak 

Lethal  4 

Tip 

Lethal  5 

Trefoil 

Lethal  6 

Truncate 

Lethal  7 

Vestigial 

Lethal  B 

Lethal  Sa 

Lethal  Sb 

Lethal  Sc 

Miniature 

Notch 

Reduplicated 

Ruby 

Rudimentary 

Sable 

Shifted 

Short 

Skee 

Spoon 

Spot 

Tan 

Truncate  intens. 

Vermilion 

White 

Yellow 

Group  IV 
Bent 
Eyeless 


MENDELIAN    SEGREGATION  7 

The  four  pairs  of  chromosomes  of  the  female  of 
Drosophila  are  shown  in  Fig.  2  (to  the  left).  There 
are  three  pairs  of  large  chromosomes  and  one  pair  of 
small  chromosomes.  One  of  the  four  is  the  pair  of  sex 
chromosomes  (X  chromosomes) .  In  the  male,  Fig. 
2  (to  the  right) ,  there  are  likewise  three  pairs  of  large 
chromosomes  and  a  smaller  pair.  The  two  sex  chro- 
mosomes in  the  male  are  here  represented  as  differ- 


FIG.  2. — Diagram  of  female  and  of  male  group  (duplex)  of  chromosomes 
of  Drosophila  ampelophila  showing  the  four  pairs  of  chromosomes.  The 
hook  on  the  Y  chromosome  is  a  convention.  The  members  of  each  pair 
are  usually  found  together,  as  here. 

ing  from  each  other  in  shape.  In  the  diagrams 
the  Y  chromosome  is  represented  as  hook  shaped, 
but  this  is  intended  only  as  a  convention.  It  is  true 
that  in  the  case  of  non-disjunction  where  the  Y 
chromosome  has  been  transferred  to  the  female  it 
has  this  hook  shape,  but  as  yet  it  has  not  been  pos- 
sible to  identify  the  Y  chromosome  as  hook  shaped 
in  the  male.  Stevens'  work  had  seemed  to  show 


8  MENDELIAN    SEGREGATION 

that  the  X  chromosome  is  attached  to  another 
chromosome  and  that  there  is  no  Y  chromosome. 
In  the  earlier  papers  on  Drosophila  this  relation  of 
the  chromosomes  was  assumed  to  be  correct  and  the 
female  was  represented  as  XX  and  the  male  as  XO. 
In  Drosophila,  then,  there  is  a  numerical  corre- 
spondence between  the  number  of  hereditary  groups 
and  the  number  of  the  chromosomes.  Moreover,  the 
size  relations  of  the  groups  and  of  the  chromosomes 
correspond.  The  method  of  inheritance  of  the 
factors  carried  by  these  chromosomes  will  now  be 
considered  more  in  detail. 

THE  INHERITANCE  OF  ONE  PAIR  OF  FACTORS 

The  inheritance  of  a  single  pair  of  characters  may 
be  illustrated  by  the  following  examples  from  Droso- 
phila, one  from  each  of  the  four  groups. 

The  mutant  stock  called  vestigial  is  so  char- 
acterized because  it  has  only  small  vestiges  of  the 
wings.  If  a  fly  with  vestigial  wings  is  mated  to  the 
wild  type  with  long  wings  (Fig.  3,  PI),  the  offspring 
will  have  long  wings  (Fig.  3,  FI).  If  these  hybrid  flies 
of  the  first  generation  (the  first  filial  generation,  or 
FI)  are  mated  to  each  other,  their  offspring  (or  F2) 
will  be  of  two  sorts:  some  will  have  long  wings  and 
others  will  have  vestigial  wings.  There  will  be  three 
times  as  many  flies  with  long  wings  as  flies  with 
vestigial  wings.  This  is  the  Mendelian  ratio  of 
3:1  that  appears  when  a  single  pair  of  characters  is 
involved. 


MENDELIAN    SEGREGATION 


VESTIGIAL 


LONG 


F, 


cmnriiD 


GAMETES 


dHHHID        SPERM 


cmmiiD 


OTTTTTTTD 


(1IIIIIIID 

FIG.  3. — Vestigial  winged  by  long  winged  (wild  type)  fly.  The  second 
chromosome  that  carries  the  recessive  factor  for  vestigial  is  cross-barred, 
the  corresponding  chromosome  of  the  normal  is  plain. 


10  MENDELIAN    SEGREGATION 

If  the  factors  for  vestigial  wings  are  carried  by  a 
pair  of  chromosomes  (the  cross-barred  chromosomes 
in  Fig.  1)  then  at  the  ripening  of  the  germ  cells  (eggs 
and  sperm)  such  a  pair  of  chromosomes  will  come 
together  (Fig.  1,  e)  and  then  separate  (Fig.  1,  g)', 
so  that  each  germ  cell  (Fig.  1,  h)  will  have  one  such 
chromosome  and  not*  the  other. 

If  such  a  sperm  cell  fertilizes  an  egg  of  the  wild  fly 
that  contains  a  similar  group  of  chromosomes,  ex- 


FIG.  4. — Diagram  to  illustrate  the  fertilization  of  an  egg  by  a  sperm 
A.  One  chromosome  in  the  egg  differs  from  the  corresponding  (ho- 
mologous )  chromosome  in  the  sperm .  The  fertilized  egg  (zygote)  with  the 
double  (duplex)  number  of  chromosomes  in  B. 

cept  that  the  corresponding  chromosome  carries  the 
factor  for  long  wings  (Fig.  4,  A),  the  result  will  be  to 
produce  a  fertilized  egg  (Fig.  4,  B)  in  which  one  mem- 
ber of  the  pair  of  chromosomes  in  question  comes 
from  the  mother  and  carries  the  factor  for  long,  and 
the  other  comes  from  the  father  and  carries  the 
factor  for  vestigial  wing.  Since  this  egg  with  both 
factors  present  produces  a  fly  with  long  wings,  the 
vestigial  character  is  said  to  be  recessive  to  the  long; 
or  conversely  the  long  is  said  to  be  dominant  to  the 
vestigial  character. 

When  the  eggs  and  the  sperm  of  hybrid  flies  of  this 
origin  come  to  maturity,  the  homologous  chromo- 


MENDELIAN    SEGREGATION 


11 


somes  conjugate  in  pairs,  as  shown  diagrammatically 
in  Fig.  5,  b.  The  chromosomes  then  separate  (Fig.  5,  c 
and  d)  at  the  time  of  division  of  the  cell ,  and  one  of  the 
resulting  daughter  cells  gets  the  chromosome  bearing 
the  vestigial,  and  the  other  daughter  cell  gets  the 
homologous  chromosome,  bearing  the  long  factor. 
Hence,  there  will  be  two  kinds  of  eggs  in  the  female 
and  two  kinds  of  spermatozoa  in  the  male.  When 
two  such  hybrid  flies  mate  with  each  other,  any 


FIG.  5. — Diagram  to  illustrate  in  a  heterozygous  individual  the  con- 
jugation and  segregation  of  the  chromosomes  during  "reduction." 

sperm  may  meet  and  fertilize  any  egg.  The  possible 
combinations  that  result,  and  the  frequency  with 
which  they  occur,  are  shown  in  the  next  diagram 
(Fig.  6). 

As  shown  in  this  diagram,  a  spermatozoon  bearing 
the  factor  for  long  wings  fertilizing  an  egg  bearing 
the  same  factor  produces  a  fly  pure  for  long  wings;  a 
spermatozoon  bearing  the  factor  for  long  wings  ferti- 
lizing an  egg  bearing  the  factor  for  vestigial  wings 
produces  a  hybrid  fly  that  has  long  wings.  Hence 
we  say  the  long  dominates  the  vestigial  character. 


12 


MENDELIAN    SEGREGATION 


Similarly,  a  spermatozoon  bearing  the  factor  for 
vestigial  wings  fertilizing  an  egg  bearing  the  factor 
for  long  wings  produces  a  hybrid  with  long  wings;  a 
spermatozoon  bearing  the  factor  for  vestigial  wings 


FIG.  6. — Diagram  to  illustrate  how  by  the  random  meeting  of  two  kinds 
of  sperm  and  two  kinds  of  eggs  the  typical  3 : 1  ratio  results. 

fertilizing  an  egg  bearing  the  factor  for  vestigial  wings 
produces  a  fly  pure  for  vestigial  wings. 

Since  the  sperm  and  the  eggs  meet  at  random  there 
should  be  1  pure  long,  to  2  heterozygous  long,  to  1  ves- 
tigial; or  putting  together  all  flies  with  long  wings, 
3  long  to  1  vestigial.  Three  to  one  is  the  character- 


MENDELIAN    SEGREGATION  13 

istic  Mendelian  ratio  when  one  pair  of  characters  is 
involved. 

In  another  mutant  stock,  ebony,  the  body  and 
wings  are  very  dark  in  contrast  to  the  wild  fly  whose 
color  is  "gray."  Gray  is  used  to  designate  the  color 
of  the  wild  fly,  whose  wings  are  gray,  but  whose 
body  is  yellowish  with  black  bands  on  the  abdomen. 
If  ebony  is  crossed  to  gray  the  offspring  (Fi)  are  gray 
but  are  somewhat  darker  than  the  ordinary  wild  flies. 
When  these  hybrids  are  inbred  they  give  (F2)  1  gray, 
to  2  intermediates,  to  1  ebony.  The  group  of  inter- 
mediates in  the  second  generation  (F2)  can  not  be 
separated  accurately  from  the  pure  gray  type.  If  they 
are  counted  as  gray,  the  result  is  three  grays  to  one 
ebony. 

Since  ebony  and  gray  assort  independently  of  long 
and  vestigial,  as  will  be  shown  later,  the  factor  for 
ebony  must  be  supposed  to  be  carried  by  a  chromo- 
some of  a  different  pair  from  the  one  that  carries 
vestigial.  Since  this  chromosome  behaves  in  the 
same  way  as  does  the  one  that  bears  the  vestigial 
factor,  the  scheme  used  for  vestigial  will  apply  here 
also. 

Another  mutant  stock  is  characterized  by  small 
eyes,  and  since  in  the  extreme  form  it  may  lack  one 
or  both  eyes  entirely  (Fig.  7),  the  name  " eyeless" 
has  been  given  to  this  mutant.  When  this  stock  is 
bred  to  wild  flies  the  offspring  have  normal  eyes. 
These  inbred  give  three  normal  to  one  eyeless  fly. 
As  shown  in  the  table  on  page  6,  this  character 
belongs  in  still  another,  the  fourth,  group,  and  its 


14 


MEN  DELI  AN    SEGREGATION 


mode  of  inheritance  is  explicable  on  the  supposition 
that  it  lies  in  the  fourth  pair  of  chromosomes. 

For  an  adequate  understanding  of  the  inheritance 


c  >»x        c1  <j 

FIG.  7. — Normal  eyes  of  Drosophila  o,  a'.  Eyeless  b-d;  b,  b'  top  and 
side  view  of  head  of  fly  without  eyes  ;c,c' right  and  left  eyes  of  another  fly ; 
d,  small  eye  on  right  side,  none  on  left. 

of  factors  in  the  first  group  it  will  be  necessary  to 
consider  the  distribution  of  the  sex  chromosomes 
(Fig.  8).  In  the  female  of  Drosophila  there  are  two 
X  chromosomes  (XX).  After  the  conjugation  and 


MENDELIAN    SEGREGATION 


15 


separation  of  the  X  chromosomes  in  the  female  there  is 
one  X  chromosome  left  in  each  egg.  In  the  male  there 
is  one  X  chromosome  and  another  chromosome,  its 
mate,  called  the  Y  chromosome.  Hence  in  the  male 
there  are  two  classes  of  spermatozoa :  one  containing 
X,  the  other  Y.  If  a  Y-bearing  spermatozoon  should 


SPERM 


GAMETES 


MALE 


FIG.   8. — Diagram   to  show  the  history  of  the  sex  chromosomes  from 
one  generation  to  the  next. 

fertilize  an  egg  the  result  will  be  an  XY  individual, 
or  male.  It  is  evident  that  the  Y  chromosome  is 
found  only  in  the  males,  while  an  X  chromosome 
passes  not  only  from  female  to  female,  but  also  from 
female  to  male  and  from  male  to  female. 

As  will  be  shown  now,  certain  factors  follow  the 
distribution  of  the  X  chromosomes  and  are  there- 


16  MENDELIAN    SEGREGATION 

fore  supposed  to  be  contained  in  them.  These  factors 
are  said  to  be  sex  linked. 

The  inheritance  of  white  eyes  may  serve  as  an 
illustration  for  the  entire  group  of  sex  linked  char- 
acters. If  a  white-eyed  male  is  bred  to  a  red-eyed 
female  (wild  type)  (Fig.  9),  the  sons  and  daughters 
(Fi)  have  red  eyes.  If  these  are  inbred  the  offspring 
(F2)  are  three  reds  to  one  white,  but  the  white-eyed 
flies  are  all  males.  If  we  trace  the  history  of  the  sex 
chromosomes  we  can  see  how  this  happens. 

In  the  red-eyed  mother,  each  egg  contains  an  X 
chromosome  bearing  a  factor  for  red  eyes.  In  the 
white-eyed  father,  half  of  the  spermatozoa  contain  an 
X  chromosome  which  carries  a  factor  for  white  eyes, 
while  the  other  half  contain  a  Y  chromosome  which 
carries  no  factors  (Fig.  9) .  Any  egg  fertilized  by  an 
X-bearing  spermatozoon  of  the  white-eyed  father  will 
produce  a  female  that  has  one  red-producing  X  chro- 
mosome and  one  white-producing  X  chromosome 
(Fig.  9).  Her  eyes  are  red,  because  red  dominates 
white.  Any  egg  fertilized  by  a  Y-bearing  spermato- 
zoon of  the  white-eyed  father  will  produce  a  son 
(Fig.  9)  that  has  red  eyes,  because  his  X  chromo- 
some brings  in  the  red  factor  from  the  mother,  while 
the  Y  chromosome  does  not  bring  in  any  dominant 
factor.  At  the  ripening  of  the  germ  cells  in  the  Fi 
female  the  number  of  chromosomes  is  reduced  to 
half.  There  result  two  kinds  of  eggs,  half  with  the 
red-bearing  and  half  with  the  white-bearing  X  (Fig. 
9).  Similarly  in  the  male  there  will  be  two  classes 
of  sperm,  half  with  the  red-bearing  X  chromosome, 


MENDELIAN    SEGREGATION 


17 


RED 


WHITE 


FIG.  9. — Red-eyed  female  by  white-eyed  male  (D.  ampelophila).     This  is 
the  reciprocal  of  the  cross  shown  in  Fig.  10. 


18  MENDELIAN    SEGREGATION 

half  with  the  indifferent  Y  chromosome.  Random 
meeting  of  eggs  and  sperm  will  give  the  result  shown 
in  the  lower  line  of  the  diagram.  There  will  be  a  3  : 1 
ratio,  as  in  other  Mendelian  crosses,  but  the  white 
individuals  in  F2  will  be  males.  The  factor  for  red  in 
the  Fi  male  will  always  stay  in  the  X  chromosome,  so 
that  all  the  female-producing  spermatozoa  will  carry 
red,  and  consequently  all  F2  females  will  be  red. 
The  males  will  have  red  eyes  if  they  receive  the  red- 
bearing  chromosome  from  their  mother  and  white 
eyes  if  they  receive  the  white-bearing  chromosome 
from  their  mother. 

The  reciprocal  cross  is  made  by  mating  a  white- 
eyed  female  to  a  red-eyed  male  (Fig.  10).  The 
daughters  will  have  red  eyes  and  the  sons  white  eyes. 
If  these  are  inbred  their  offspring  will  be  red  and 
white  in  equal  numbers,  and  not  the  usual  three 
reds  to  one  white.  The  explanation  of  this  new 
ratio  is  at  once  apparent  as  soon  as  the  history  of  the 
sex  chromosomes  is  studied. 

The  two  X  chromosomes  in  the  white-eyed  mother 
carry  the  factor  for  white  eyes.  After  ripening,  each 
egg  carries  one  white-bearing  X  chromosome.  The 
single  X  chromosome  of  the  female-producing  sper- 
matozoon of  the  red-eyed  father  carries  the  factor  for 
red  eyes ;  the  male-producing  spermatozoa  carry  the  Y 
chromosome  which,  as  stated  above;  is  indifferent. 
Any  egg  fertilized  by  a  spermatozoon  containing  the 
red-bearing  X  chromosome  will  produce  a  red  daugh- 
ter, because  red  dominates  white.  Conversely,  any 
egg  fertilized  by  the  Y-bearing  male-producing  sper- 


MENDELIAN    SEGREGATION 


19 


WHITE 


FIG.  10. — White-eyed  female  by  red-eyed  male  (D.  ampelophila). 
The  factors  for  these  characters  are  carried  by  the  X  chromosomes,  the 
factor  for  red  by  the  black  X,  and  the  factor  for  the  white  by  the  plain  X. 
The  history  of  the  chromosomes  is  shown  in  the  middle  of  the  diagram. 


20  MENDELIAN    SEGREGATION 

matozoon  will  produce  a  white-eyed  son,  because  the 
only  X  chromosome  that  the  son  contains  is  derived 
from  his  mother,  both  of  whose  X  chromosomes  carry 
a  white-producing  factor. 

When  these  red-eyed  daughters  and  white-eyed 
sons  are  inbred  the  possible  combinations  are  shown 
in  the  lower  line  of  the  diagram  (Fig.  10). 

There  will  be  two  kinds  of  eggs,  one  containing  a  red- 
bearing,  the  other  a  white-bearing,  X  chromosome. 
The  female-producing  spermatozoa  will  contain  a 
white-bearing  X  chromosome;  the  male-producing 
spermatozoa  will  contain  a  Y  chromosome.  A  red- 
bearing  egg  fertilized  by  a  female-producing  sper- 
matozoon will  produce  a  red-eyed  female;  a  white- 
bearing  egg  fertilized  by  a  female-producing  spermato- 
zoon will  produce  a  white-eyed  female.  A  red-bear- 
ing egg  fertilized  by  a  male-producing  spermatozoon 
will  produce  a  red-eyed  male;  a  white-bearing  egg 
fertilized  by  a  male-producing  spermatozoon  will 
produce  a  white-eyed  male.  The  resulting  ratio  is 
1  red  to  1  white,  in  both  sexes. 

The  distribution  of  the  chromosomes  explains  how 
in  one  cross  the  Mendelian  ratio  of  3  : 1  obtains,  and 
also  how  in  the  reciprocal  cross  there  is  a  1 : 1  ratio. 

THE  INHERITANCE  OF  Two  OR  MORE  INDEPENDENT 
PAIRS  OF  FACTORS 

The  application  of  the  chromosome  hypothesis 
to  crosses  between  races  that  differ  in  two  pairs  of 
factors  is  illustrated  by  the  following  example  (Fig. 


MENDELIAN    SEGREGATION  21 

11).  If  a  vestigial  gray  fly  is  mated  to  a  long-winged 
ebony  fly,  all  the  offspring  (Fi)  will  have  long  wings 
and  gray  (or  slightly  darker)  body  color.  If  these 
hybrids  (Fi)  are  inbred,  offspring  (F2)  will  be  pro- 
duced in  the  ratios: 

9  Flies  with  long  wings  and  gray  body  color. 
3  Flies  with  vestigial  wings  and  gray  body  color. 
3  Flies  with  long  wings  and  ebony  body  color. 
1  Fly  with  vestigial  wings  and  ebony  body  color. 

In  the  diagram  (Fig.  11)  two  pairs  of  chromosomes, 
the  second  and  the  third  pairs,  are  represented  by  the 
following  conventions:  The  cross-barred  chromo- 
somes, each  of  wThich  carries  a  factor  for  vestigial, 
are  the  second  pair.  The  third  pair,  that  contains 
the  factors  for  ebony,  is  represented  as  black.  The 
third  pair  of  chromosomes  in  the  vestigial  fly  is 
"normal"  in  respect  to  ebony.  Correspondingly  the 
second  pair  of  chromosomes  in  the  ebony  fly  is 
"normal"  in  respect  to  vestigial. 

Each  germ  cell  of  the  vestigial-gray  parent  will 
contain  one  chromosome  with  the  factor  for  vestigial 
and  one  for  gray,  and  each  germ  cell  of  the  long- 
winged  ebony  parent  will  contain  one  chromosome 
with  the  factor  for  long  and  one  for  ebony.  The 
hybrid  (Fig.  11)  will  contain,  therefore,  a  pair 
of  chromosomes,  one  of  which  carries  vestigial,  the 
other  long;  and  will  also  contain  another  pair,  one  of 
which  carries  ebony,  the  other  gray. 

In  the  maturation  of  the  germ  cells  of  the  hybrid, 
the  members  of  each  pair  separate  from  each  other 
as  shown  in  Fig.  11  in  the  gametogenesis  of  Fi. 


22 


MEXDELIAN    SEGREGATION 


(mTTTTTD 
(TimTTTD 

VESTIGIAL        GRAV 


LONG 


EBONY 


GRAY 


(UIIIIIID 


(LJIIIIIID 


GAMETOGENESIS  orF^ 

FIG.  11. — Vestigial  gray  by  long  ebony  fiy,  to  illustrate  the  inheritance 
of  two  pairs  of  characters.  The  factor  for  vestigial  is  carried  by  the 
second,  the  factor  for  ebony  by  the  third  chromosome  pair.  In  the  lower 
part  of  the  figure  the  two  modes  of  separation,  of  the  two  pairs  of  chro- 
mosomes involved  here,  are  represented.  Four  kinds  of  gametes  result. 
These  four  kinds  combine  at  random  in  fertilization,  so  that  16  classes 
are  produced  in  F2,  as  shown  in  the  next  figure. 


MENDELIAN    SEGREGATION 


23 


The  two  pairs  of  chromosomes  " assort"  on  the 
spindle  in  either  one  of  the  two  ways  shown  in  the 
diagram;  resulting  in  four  and  only  four  kinds  of 
gametes. 


EGGS 


SPERM 


ammii)  CUD 

iliilllll  4Hto 

LONG             GRAY 

LONG           CRAY 

LONG           GRAY 

LONG           GRAY 

CZZD    C            "3 

ss 

<nnnm>  CUD 

(HUMP  4B9 

LONG            CRAY 

LONG              EBONY 

LONG           GRAY 

LONG              EBONY 

(rnriTTTtic     ~> 

amnnD  c      3 

annED  c     -) 

amMD  C      "3 

anHED  •to 
<anniiD  CZZD 

LONG           GRAY 

LONG             CRAY 

VESTIGIAL       GRAY 

VESTIGIAL      GRAY 

annniD  €•» 

amnnD  ••* 

(ffllllilD  CUD 

(IHLTIEP  4to 

annniD  ^H^ 
annnD)  «i^p 

LONG         CRAY 

LONG             EBONY 

VESTIGIAL       CRAY 

VESTIGIAL      EBONY 

FIG.  12. — Diagram  to  show  the  16  possible  kinds  of  permutations  of 
the  four  kinds  of  gametes  of  Fig.  11.  Along  the  top  line  are  four  kinds 
of  eggs ;  along  the  left  side  are  four  kinds  of  sperm ;  in  the  squares  are  the 
combinations  formed  by  the  meeting  of  each  kind  of  egg  with  each  kind 
of  sperm,  giving  9  long  gray;  3  long  ebony;  3  vestigial  gray;  1  vestigial 
ebony. 

The  process  just  described  takes  place  both  in  the 
male  and  in  the  female.  Consequently  there  will 
be  four  kinds  of  eggs  and  four  kinds  of  spermatozoa. 


24  MENDELIAN   SEGREGATION 

Chance  meeting  between  these  will  give  the  results 
shown  in  the  next  diagram  (Fig.  12). 

In  the  table  (Fig.  12)  the  four  kinds  of  eggs  are 
represented  at  the  head  of  the  four  vertical  columns, 
and  the  four  kinds  of  spermatozoa  at  the  left  of  each 
horizontal  row.  In  the  squares  the  combination  of 
each  kind  of  sperm  with  each  kind  of  egg  is  repre- 
sented, giving  the  ratio  of  9  long  gray:  3  vestigial 
gray :  3  long  ebony :  1  vestigial  ebony. 

The  F2  expectation  may,  of  course,  be  derived  more 
directly  as  follows:  There  wrill  be  3  long  to  1  ves- 
tigial. These  longs  wrill  be  both  gray  and  ebony  in 
the  ratio  again  of  3  to  1 ;  hence  9  long  gray  to  3  long 
ebony.  Correspondingly,  the  vestigials  will  be  both 
gray  and  ebony,  in  the  ratio  of  3  to  1;  hence  3 
vestigial  gray  to  1  vestigial  ebony.  The  result  is  the 
same  as  before. 

If  one  of  two  independent  pairs  of  characters  is 
sex  linked,  the  same  scheme  holds  in  those  cases  where 
the  recessive  sex  linked  character  enters  through  the 
grandfather,  but  the  ratio  is  different  when  the  re- 
cessive sex  linked  character  enters  through  the 
grandmother  (viz.,  3  :3  :1  :1),  as  is  to  be  expected 
from  the  mode  of  inheritance  of  white  eyes  taken 
alone  j1  and  here,  too,  the  result  conforms  fully  to  the 
chromosome  scheme. 

Three  factors  can  be  worked  out  by  means  of  the 

1  For  example,  taking  white  and  red  alone  the  ratio  of  the  F2  is 
1:1.  But  among  the  reds  the  ratio  of  gray  to  ebony  will  be  3  : 1  and 
among  the  whites  will  be  3  : 1.  Hence  the  result  3  red  gray,  1  red  ebony, 
3  white  gray,  1  white  ebony. 


MENDELIAN    SEGREGATION 


25 


chromosomes  as  readily  as  one  or  two.  It  will  not 
be  necessary  to  give  the  full  analysis,  for  it  will  be 
easily  understood  from  the  scheme  already  given. 
If  a  fly  with  vestigial  wrings  is  crossed  to  an  ebony, 
eyeless  fly  three  pairs  of  factors  are  involved  that  lie 
in  different  chromosomes.  The  FI  flies  are  normal, 
for  there  is  in  the  hybrid  a  normal  mate  for  each  of 
the  three  recessive  factors.  The  possible  recombina- 
tions are  shown  in  the  next  diagram,  Fig.  13.  There 


amnnD 


aiiiiiiiD 


OJIIIIIID 


FIG.  13. — Diagram  to  show  the  segregation  of  the  three  pairs  of  chro- 
mosomes. Eight  combinations  are  possible,  giving  8  kinds  of  germ  cells, 
with  64  possible  re-combinations. 

are  four  different  positions  for  the  chromosome  pairs 
on  the  spindle,  leading  to  eight  kinds  of  germ  cells. 
By  chance  meetings  of  the  eight  kinds  of  sperm  with 
the  eight  kinds  of  eggs  there  will  result  8  types  as 
follows : 

27  Long,  gray,  normal  eye  (wild  type). 

9  Vestigial,  gray,  normal  eye. 

9  Long,  ebony,  normal  eye. 

9  Long,  gray,  eyeless. 

3  Vestigial,  ebony,  normal  eye. 

3  Vestigial,  gray,  eyeless. 

3  Long,  ebony,  eyeless. 

1  Vestigial,  ebony,  eyeless. 


26  MENDELIAN    SEGREGATION 

The  same  manner  of  treatment  will  work  for  more 
than  three  pairs  of  chromosomes;  the  number  of 
kinds  of  germ  cells  increases  in  geometrical  ratio. 
In  most  animals  and  plants  the  number  of  chromo- 
somes is  higher  than  in  Drosophila,  and  the  number 
of  pairs  of  factors  that  may  show  independent  assort- 
ment is,  in  consequence,  increased.  In  the  snail, 
Helix  hortensis,  the  half  number  of  the  chromosomes 
is  given  as  22;  in  the  potato  beetle  18;  in  man,  prob- 
ably, 24;  in  the  mouse  20;  in  cotton  28;  in  the  four- 
o'clock  16;  in  the  garden  pea  7;  in  corn  20;  in  the 
evening  primrose  7;  in  the  nightshade  36;  in  tobacco 
24;  in  the  tomato  12;  in  wheat  8.  If  20  pairs  of 
chromosomes  are  present  there  will  be  over  one 
million  possible  kinds  of  germ  cells  in  the  FI  hybrid. 
The  number  of  combinations  that  two  such  sets  of 
germ  cells  may  produce  through  fertilization  is 
enormously  greater.  From  this  point  of  view  we 
can  understand  the  absence  of  identical  individuals 
in  such  mixed  types  as  the  human  race.  The  chance 
of  identity  is  still  further  decreased  since  in  addition 
there  may  be  very  large  numbers  of  factors  within 
each  chromosome. 


CHAPTER  II 
TYPES  OF  MENDELIAN  HEREDITY 

Experience  has  shown  that  Mendelian  inheritance 
applies  to  all  sorts  of  characters,  structural,  physio- 
logical, pathological,  and  psychological;  to  characters 
peculiar  to  the  egg,  to  the  young,  and  even  to  old 
age;  to  length  of  life;  to  fundamental  taxonomic 
characters  as  well  as  to  "superficial"  characters;  and 
to  characters  intimately  concerned  in  maintaining 
the  life  of  the  individual,  as  well  as  to  characters  which 
apparently  do  not  influence  survival.  Some  of  these 
different  types  and  their  mode  of  inheritance  will  be 
briefly  described,  but  since  the  general  principles  in- 
volved are  more  important  than  the  kind  of  character 
that  is  affected,  the  results  will  be  treated  under 
general  headings. 

DOMINANCE  AND  RECESSIVENESS 

The  four-o'clock  (Mirabilis  jalapa)  has  a  white  and 
a  red-flowered  variety.  If  these  are  crossed  the  hy- 
brid is  pink  in  color.  The  pink  hybrid  inbred  (self- 
fertilized  in  this  case)  gives  in  the  next  generation 
(F2)  one  red,  to  two  pink,  to  one  white  (Fig.  14). 
Owing  to  the  intermediate  color  of  the  hybrid  (or 
heterozygote)  it  is  impossible  to  say  that  either 
color  dominates  the  other.  The  factor  for  red  and 

27 


2S 


TYPES    OF    MENDELIAN    HEREDITY 


the  factor  for  white  both  affect  the  plant  in  which  they 
occur.  In  this  and  in  similar  cases  the  F2  ratio  of 
1 :2  : 1  is  obtained,  because  it  is  possible  to  distinguish 
the  pure  red  and  the  pure  white  from  the  heterozygous 
plants. 


PARENTS 


FIG.  14. — Diagram  to  illustrate  the  cross  between  a  red  and  a  white 
flowered  Mirabilis  jalapa  (4  o'clock),  which  produces  a  pink,  intermediate 
heterozygote. 

The  Andalusian  fowl  is  a  similar  case.  When 
certain  races  of  black  are  bred  to  certain  races  or 
kinds  of  " white"  the  hybrid  is  slate  "blue"  in  color. 
These  blue  birds,  called  Andalusians,  when  inbred, 
give  one  black  to  two  blue  to  one  white.  Blue  is 


TYPES    OF    MENDELIAN    HEREDITY 


29 


the  heterozygous  condition;  it  is  not  possible  to 
produce  a  pure  breeding  race  of  Andalusians,  for  the 
combination  that  produced  an  Andalusian  falls  apart 
in  the  germ  cells  of  the  Andalusian  birds.  The  bird  is 
blue  because  the  pigment  is  not  spread  evenly  over 
the  feather  but  is  restricted  to  small  but  black  specks. 


FIG.  15. — Normal  (a,  a')  and  bar  eye  (6,  6')  of  Drosophila;  shown  in 
side  view,  and  as  seen  from  above. 

The  Andalusian  blue  is  a  mosaic  of  black  and  white, 
and  not  at  all  a  dilute  black. 

A  good  example  of  an  intermediate  hybrid  is  found 
when  the  mutant  fly  with  bar  eye  (Fig.  15)  is  bred  to  a 
wild  fly.  The  daughters  have  bar  eyes  that  are  not 
as  narrow  as  those  of  the  pure  bar  stock.  The  range 
of  variation  is  great,  however,  for  some  of  the  hybrids 
have  eyes  that  are  nearly  as  round  as  the  normal,  and 


30  TYPES    OF    MENDELIAN    HEREDITY 

in  others  the  eye  is  nearly  as  narrow  a  bar  as  that  of 
pure  stock.  In  the  male,  which  has  one  factor  for 
bar  eye,  the  eye  is  as  narrow  as  in  the  pure  (i.e., 
homozygous)  female  with  two  factors.  The  inter- 
mediate condition  in  the  female  which  is  hybrid 
(heterozygous)  for  this  factor  is  hence  not  explained 
by  the  lesser  effect  of  the  single  factor,  but  is  probably 
due  to  the  competing  influence  of  the  other  allelo- 
morph. Of  course  it  might  be  contended  that  since 
in  the  male  there  is  a  different  chromosome  complex 
(XABCD  YABCD)  from  that  in  the  female  (XABCD- 
XABCD)  it  is  this  difference  in  other  factors  that 
causes  the  heterozygous  female  to  have  a  wider  eye 
than  the  male;  but  this  argument  is  rendered  improb- 
able here,  when  we  recall  that  in  only  one  out  of  many 
cases  of  sex  linked  inheritance,  in  which  the  hetero- 
zygous female  is  intermediate,  is  the  male  different 
from  the  homozygous  female. 

In  other  cases  the  influence  of  one  of  the  parents 
of  the  cross  may  be  so  slight  as  to  escape  detection 
on  ordinary  observation,  and  may  require  special 
measurements  for  demonstration.  When  flies  with 
miniature  wings  (Fig.  16)  are  mated  to  wild  flies, 
the  daughters  have  long  wings,  which  Lutz  has  shown 
to  be  a  little  shorter  in  proportion  to  the  length  of 
the  legs  than  are  the  wings  of  wild  females;  but  the 
difference  is  so  slight  that  it  could  not  have  been 
detected  without  biometrical  methods. 

Finally,  we  must  consider  the  class  of  cases  in 
which  complete  dominance  has  been  described.  All 
the  cases  given  by  Mendel  in  peas  were  supposed 


TYPES    OF    MENDELIAN    HEREDITY 


31 


to  fall  under  this  heading :  yellow  dominates  green, 
round  dominates  wrinkled,  etc. 

Whether  a  character  is  completely  dominant  or  not 
appears  to  be  a  matter  of  no  special  significance. 
In  fact  the  failure  of  many  characters  to  show  complete 
dominance  raises  a  doubt  as  to  whether  there  is  such 


FIG.  16. — a,  Long   wing  (wild  type)  of  Drosophila;  b,  miniature  wing. 
(a  and  b  are  not  drawn  to  scale.) 

a  condition  as  complete  dominance.  Some  cases  ap- 
proach so  nearly  to  that  condition  that  special  tests 
may  be  required  to  show  that  the  hybrid  is  affected 
by  the  recessive  factor.  For  instance,  in  flies  the 
factor  for  white  eyes  seems  to  produce  no  effect 
when  white  is  bred  to  red.  The  Fi  reds  are  indis- 
tinguishable from  pure  reds.  But  by  weakening  the 
red  by  adding  recessive  factors  other  than  white, 
the  influence  of  white  can  be  demonstrated,  as  Mor- 


32  TYPES    OF    MENDELIAN    HEREDITY 

gan  and  Bridges  have  shown.  Therefore  although  the 
effect  of  the  white  factor  can  not  be  detected  in  the 
single  combination  with  red,  it  is  reasonable  to  sup- 
pose that  some  effect  is  really  present.  Similarly, 
conditions  were  found  in  which  the  effect  of  hetero- 
zygosis  for  eosin,  vermilion,  or  pink  could  be  demon- 
strated. While  the  question  is  one  of  only  sub- 
sidiary importance,  yet  in  the  separation  of  classes 
it  is  often  useful  to  be  able  to  distinguish  the  pure 
from  the  hybrid  form ;  but  whether  this  can  or  can  not 
be  done  in  any  given  case  does  not  affect  the  funda- 
mental principle  of  segregation  which  is  the  essential 
feature  of  Mendel's  discovery. 

MANIFOLD  EFFECTS  OF  SINGLE  FACTORS 

It  is  customary  to  speak  of  a  particular  character 
as  the  product  of  a  single  factor,  as  though  the  factor 
affected  only  a  particular  color,  or  structure,  or  part 
of  the  organism.  But  everyone  familiar  at  first  hand 
with  Mendelian  inheritance  knows  that  the  so-called 
unit  character  is  only  the  most  obvious  or  most  sig- 
nificant product  of  the  postulated  factor.  Most 
students  of  Mendelian  heredity  will  freely  grant  that 
the  effects  of  a  factor  may  be  far-reaching  and 
manifold.  A  few  examples  may  make  this  plain. 

In  Drosophila  there  is  a  mutant  stock  called 
"club,"  in  which  the  wing  pads  fail  to  unfold  (Fig.  17) 
in  about  20  per  cent,  of  the  flies.  In  the  majority  of 
club  flies  the  wings  expand  fully,  and  are  like  those 
of  the  wild  fly.  Owing  to  this  fact,  that  not  all  the 


TYPES    OF    MENDELIAN    HEREDITY 


33 


flies  even  in  a  pure  stock  of  club  show  this  character, 
it  was  difficult  to  study  the  inheritance  of  the 
supposed  factor  that  sometimes  inhibits  the  unfolding 
of  the  wing  pads.  Nevertheless,  it  was  possible 
even  with  this  handicap  to  show  that  the  character 
depended  on  a  sex  linked  recessive  factor.  Later 


FIG.  17. — Club  wing  (to  left).  The  absence  of  the  spines  on  the  side 
of  the  thorax  in  "club"  is  shown  in  c,  and  the  normal  condition  is  shown 
in  b. 

the  discovery  was  made  that  a  particular  pair  of 
spines  always  present  on  the  side  of  the  thorax  of 
the  wild  flies,  is  absent  from  the  club  flies,  irrespective 
of  whether  the  wings  do  or  do  not  unfold  (Fig.  17,  c). 
This  constant  feature  of  the  mutant  made  its  study 
quite  simple.  Another  pair  of  spines,  those  upon  the 


34 


TYPES    OF    MENDELIAN    HEREDITY 


rear  margin  of  the  scutellum,  point  constantly  in  an 
abnormal  direction  in  club  stock.  The  head  of  club 
flies  is  often  flattened,  the  eyes  are  smaller,  and  the 
thorax  and  abdomen  are  somewhat  distorted.  Here 
we  have  an  example  of  a  single  germinal  difference, 
the  factor  for  club,  producing  several  distinct  effects, 


FIG.  18. — Rudimentary  wing  (to  left),  and  truncate  wing  (to  right). 

some  of  which  are  constant  features  of  the  stock,  while 
others  are  occasional  or  variable. 

Another  and  similar  example  is  found  in  the  rudi- 
mentary winged  flies  (Fig.  18,  a) .  The  wing  is  usually 
shorter  than  the  abdomen,  but  may  be  longer  and  even 
approach  the  normal  wing  in  length  and  shape.  The 


TYPES    OF    MENDELIAN    HEREDITY  35 

last  pair  of  legs  are  often  thicker  and  shorter.  If 
many  larvae  are  present,  or  the  food  conditions  poor, 
the  larvseof  rudimentary  flies  can  not  stand  the  compe- 
tition and  die  off ,  and  in  consequence  the  rudimentary 
class  is  smaller  than  expected.  The  males  are  fertile, 
but  the  females  are  almost  entirely  sterile,  although 
rarely  one  of  them  may  lay  a  few  eggs  and  some  of 
these  hatch.  The  infertility  is  probably  due  to  ab- 
sence or  rareness  of  mature  eggs  in  the  ovaries. 
There  are  also  other  effects  than  these  four  men- 
tioned, all  of  which  are  produced  by  the  same  factor, 
and,  no  doubt,  were  our  knowledge  complete,  we 
should  find  in  all  mutants  many  differences  in  addi- 
tion to  the  ones  picked  out  for  study  and  called  "unit 
characters."  DeVries'  definition  of  mutation  en- 
tirely covers  this  relation;  in  fact,  it  even  goes 
further  and  implies  that  a  single  difference  may 
affect  the  entire  organization.  Perhaps  this  does 
occur,  but  practically  the  number  of  differences  that 
can  be  observed  between  a  wild  and  a  mutant  stock 
derived  from  it,  is  limited.  The  attack  that  is  some- 
times made  on  the  unit  character  hypothesis  fails  in 
its  intention  the  moment  it  is  understood  that  a 
single  factor  (difference)  has  generally  not  one  but 
many  effects.  Most  workers  in  Mendelian  heredity 
are  fully  conversant  with  these  facts.  This  attack 
on  the  unit  character  conception  is  usually  made 
by  those  not  familiar  with  the  actual  situation  and 
who  take  the  expression  unit  character  too  literally. 
It  may  be  conceded  that  the  expression  has  at  times 
been  abused  even  by  some  of  Mendel's  followers. 


36  TYPES  OF  MENDELIAN  HEREDITY 

SIMILAR  EFFECTS  PRODUCED  BY  DIFFERENT  FACTORS 

There  are  many  cases  in  which  characters  that  are 
superficially  alike  are  the  product  of  different  factors. 
White  color  that  characterizes  so  many  domesticated 
races  of  plants  and  animals  is  a  case  in  point.  There 
are  two  pure  breeding  races  of  white  flowered  sweet 
peas.  When  crossed,  they  produce  colored  flowers. 
When  the  FI  offspring  are  inbred  the  F2  generation 
consists  of  9  reds  to  7  whites.  This  9  : 7  ratio  is  a 
special  case  of  the  9  :3  :3  :1,  in  which  the  last  three 
classes  are  superficially  alike.  The  explanation  here 
is  that  there  are  two  kinds  of  recessive  whites  that 
have  originated  independently.  On  the  chromo- 
some hypothesis  one  white  is  due  to  mutation  in  one 
chromosome  and  the  other  white  to  mutation  in  an- 
other chromosome.  When  the  races  are  crossed, 
each  race  supplies  that  chromosome  which  contains 
the  normal  factor  of  the  white  of  the  other  race. 
In  the  F2  generation  any  plant  that  contains  at  least 
one  of  the  normal  chromosomes  of  both  pairs  will 
not  be  white.  There  will  be  nine  such  cases.  Any 
plant  that  contains  both  of  the  white-producing  chro- 
mosomes of  either  pair  will  be  white.  There  will  be 
seven  such  cases. 

There  are  also  two  pure  races  of  white  fowls  that, 
when  crossed,  give  colored  birds.  Each  white 
behaves  as  a  recessive  to  color.  For  instance,  the 
white  silky  crossed  to  a  white  dorking  gives  colored 
birds.  These  inbred  give  9  colored  to  7  white 
birds. 


TYPES    OF    MENDELIAN    HEREDITY  37 

There  is  a  third  kind  of  white  race  of  poultry, 
namely,  white  Leghorn,  in  which  white  is  dominant. 
Crossed  to  colored  birds  the  offspring  are  white 
(with  often  a  few  colored  feathers,  which  indicates 
that  dominance  is  not  complete). 

In  the  silkworm  also  a  dominant  white  and  a  reces- 
sive white  factor  have  been  found.  The  genetic 
results  are  comparable  in  all  respects  to  those  in  the 
fowi. 

There  are  also  cases  of  blacks  or  melanic  types, 
that  have  different  factorial  bases.  There  are  three 
black  races  of  Drosophila — called  sable,  black,  and 
ebony — that  belong  respectively  to  the  first,  second, 
and  third  groups.  These  are  much  alike,  but  close 
scrutiny  reveals  slight  differences.  Any  two  crossed 
together  give  gray  FI  flies. 

There  are  three  pink  eye  colors  in  Drosophila,  one 
whose  locus  is  in  the  third  chromosome  (pink),  and 
two  sex  linked  eye  colors  which  are  so  similar  that  no 
certain  difference  between  them  can  be  observed. 

Not  only  pigment  but  also  structural  characters 
may  parallel  each  other  in  a  remarkable  manner.  For 
example,  in  Drosophila  the  mutant  stocks  "bow" 
(sex  linked)  and  "arc"  (II  chromosome)  have  wings 
that  curve  evenly  downward  over  the  abdomen. 
There  are  also  two  kinds  of  flies  whose  wings  turn 
up  sharply  near  the  ends.  These  stocks  are  ' '  j  aunty ' ' 
(second  chromosome)  and  "jaunty  I,"  which  is  sex 
linked.  Two  types,  called  "fringed"  (II  chromosome) 
and  "spread"  (III  chromosome),  are  characterized 
by  thin  textured  wings  held  out  nearly  at  right 


38  TYPES    OF    MENDELIAN    HEREDITY 

angles  to  the  body.  In  the  case  of  rudimentary  and 
truncate  (Fig.  18)  the  wings  are  so  similar  that 
without  breeding  tests  one  of  them  might  easily  be 
taken  for  the  other.  Finally,  "facet"  and  "rough" 
both  have  the  ommatidia  of  the  eye  disarranged  very 
much  in  the  same  way. 

MODIFICATION  OF  THE  EFFECTS  OF  FACTORS 
/.  By  Environmental  Influences 

It  is  a  commonplace  that  the  environment  is  es- 
sential for  the  development  of  any  trait,  and  that 
traits  may  differ  according  to  the  environment  in 
which  they  develop.  In  most  cases  different  genetic 
types  produce  different  results  in  any  ordinary 
environment.  The  environment,  being  common  to 
the  two,  may  therefore  in  such  cases  be  ignored, 
or  rather  taken  for  granted.  There  are  other  cases, 
however,  in  which  a  particular  genetic  type  appears 
different  from  another  one  only  in  a  special  environ- 
ment. Where  this  environment  is  not  the  normal 
one,  its  discovery  is  an  essential  element  of  the 
experiment. 

One  of  the  best  cases  is  that  given  by  Baur.  The 
red  primrose  (Primula  sinensis  rubra)  reared  at  a  tem- 
perature of  30°-35°  C.  (with  moisture  and  shade) 
has  pure  white  flowers,  but  the  same  plants  reared  at 
15°-20°  have  red  flowers.  If  the  white-bearing  plants 
are  brought  into  a  cooler  place,  the  flowers  that  are 
already  in  bloom  remain  white,  but  those  that  de- 
velop later  in  the  cooler  temperature  are  red.  There 


TYPES  OF  MENDEL1AN  HEREDITY        39 

is  another  race  of  primula  (Primula  sinensis  alba) 
that  always  has  white  flowers,  even  at  20°.  Strictly 
speaking,  we  should  say,  not  as  we  generally  do  for 
brevity's  sake,  that  the  difference  between  the 
two  races  is  that  one  has  white,  the  other  red  flowers, 
but  we  should  say  rather  that  P.  rubra  reacts  at  20° 
by  producing  red,  at  30°  by  forming  white  flowers; 
P.  alba,  on  the  other  hand,  reacts  both  at  20°  and  at 
30°  by  producing  white  flowers.  The  constant  dif- 
ference between  these  races  is  not  in  their  color,  but  in 
the  possibility  of  producing  specific  colors  at  certain 
temperatures. 

This  is  the  point  of  view,  of  course,  that  must  also 
be  taken  for  cases  in  which  differences  exist  in  all  the 
usual  environments;  for,  here  also,  it  is  the  different 
possibilities  of  reaction  that  are  inherited.  Brevity 
warrants  us  in  speaking  of  particular  characters  as 
inherited,  rather  than  the  specific  possibility  of  reac- 
tion that  gave  these  characters;  but  no  one  need  be 
misled  by  the  shorter  expression. 

Two  similar  cases  of  the  influence  of  the  environ- 
ment have  been  found  in  Drosophila.  There  is  a 
mutant  stock  known  as  abnormal  abdomen  in  which 
the  normal  black  bands  of  the  abdomen  are  broken 
and  irregular  or  even  entirely  absent  (Fig.  19) .  In  flies 
reared  on  moist  food  the  abnormality  is  extreme; 
but  even  in  the  same  culture  the  flies  that  continue 
to  hatch  become  less  and  less  abnormal  as  the  culture 
becomes  more  dry  and  the  food  scarce,  until  finally 
the  flies  that  emerge  later  can  not  be  told  from  normal 
flies.  If  the  culture  is  kept  well  fed  the  change  does 


40 


TYPES    OF    MEXDELIAX    HEREDITY 


not  occur,  but  if  the  flies  are  reared  on  dry  food  they 
are  normal  from  the  beginning.  The  character  is  a 
sex  linked  dominant,  as  shown  by  the  following 
crosses.  When  an  abnormal  male  is  bred  to  a  normal 
(wild)  female,  the  daughters  are  abnormal  (if  the 


FIG.  19. — Mutant  type  called  Abnormal  Abdomen  of  Drosophila 
ampelophila  (the  wings  have  been  cut  off) ;  a  is  female;  b,  male;  c.  female 
that  approaches  the  normal  type. 

food  is  moist) ,  but  all  the  sons  are  normal.  If  the 
medium  is  dry,  however,  both  the  daughters  and  the 
sons  alike  are  normal.  But  these  normal  F  i  daughters 
will  produce  the  expected  abnormal  offspring  if  the 
conditions  are  suitable,  and  these  offspring  are  just  as 


TYPES    OF    MENDELIAN    HEREDITY  41 

abnormal  as  though  the  female  had  herself  been  abnor- 
mal. The  reciprocal  cross,  viz.,  abnormal  females  by 
normal  males,  gives  abnormal  sons  and  daughters,  if  the 
food  is  suitable,  but  normal  if  the  food  is  dry,  etc.  In 
both  cases  the  F2  gives  the  expectation  for  a  sex-linked 
dominant  factor  if  the  medium  is  suited  to  bring  out 
the  abnormal  character,  and  the  result  is  entirely  ob- 
scured if  the  food  is  dry.  Here,  at  will,  we  can  demon- 
strate a  regular  Mendelian  ratio  by  control  of  the 
environment,  and  conversely,  we  can  conceal  com- 
pletely what  is  taking  place  by  substituting  another 
environment.  That  the  same  genetic  process  is  going 
on  in  both  cases  can  be  demonstrated  by  suitable 
tests. 

A  case  similar  in  principle  occurs  in  a  mutant  stock 
of  Drosophila  that  produces  supernumerary  legs. 
This  stock  was  observed  in  winter  to  produce  a  con- 
siderable percentage  of  flies  with  supernumerary  legs, 
but  few  or  none  in  summer,  especially  in  warm 
weather.  Miss  Hoge,  who  has  studied  this  stock, 
finds  that  when  the  flies  are  kept  in  an  ice  chest  at  a 
temperature  about  10°  C.  a  high  percentage  of  flies 
with  supernumerary  legs  occurs.  Sometimes  several 
legs  or  parts  of  a  leg  are  doubled,  or  the  doubling 
may  occur  twice  in  the  same  leg.  The  general  rule 
that  Bateson  pointed  out  for  duplicated  legs  in  other 
insects  appears  to  hold  here,  viz.,  the  adjacent  parts 
are  mirror  images  of  each  other. 

In  the  cold  the  duplicate  leg  gives  a  regular 
Mendelian  result;  but  at  normal  temperature  the 
duplication  is  a  rare  event  and  its  mode  of  inheritance 


42  TYPES    OF    MENDELIAN    HEREDITY 

obscured.  In  a  hot  climate  there  would  be  no  evi- 
dence that  such  a  factor  was  being  regularly  trans- 
mitted. But  if  the  type  moved  into  a  cold  region 
it  would  show  duplication  in  many  of  the  legs. 

//.  By  Developmental  Influences 

"Age,"  too,  is  in  a  sense  an  environmental  condi- 
tion, which  influences  the  development  of  characters. 
Thus  a  white  flower  may  change  to  purple  as  the  plant 
gets  older,  or  the  flaxen  hair  of  a  child  may  turn  to 
brown  when  he  becomes  a  man.  But,  as  in  the  case 
of  other  "environmental"  conditions,  age  may  not 
have  the  same  effect  on  individuals  with  different 
factors;  in  this  way  it  comes  about  that  animals  or 
plants  which  differ  by  certain  factors  may  show  a 
difference  in  character  only  at  certain  ages,  or  may 
not  show  the  same  difference  at  all  ages.  In  Droso- 
phila,  flies  with  the  factor  for  pink  eyes  are  easily 
distinguishable  from  those  with  the  factor  for  purple 
eyes,  when  the  flies  are  young,  but  as  they  grow  older, 
the  eyes  of  both  races  assume  a  dark  purplish  shade, 
and  become  practically  indistinguishable  from  each 
other.  Conversely,  old  flies  with  the  factor  for  black 
are  usually  easy  to  separate  from  those  having  the 
normal  "gray"  factor,  but  the  newly  hatched  flies, 
in  which  the  black  pigment  is  not  yet  fully  developed, 
are  separated  with  greater  difficulty. 

These  cases  in  which  a  factor-difference  has  a  visible 
effect  only  at  a  certain  age  are  in  no  fundamental 
respect  different  from  cases  like  that  of  the  Drosophila 


TYPES  OF  MENDELIAN  HEREDITY        43 

with  reduplicated  legs,  where  a  factor  difference  has  a 
visible  effect  only  under  special  external  circum- 
stances. 

A  number  of  cases  of  Mendelian  inheritance  are 
known  in  which  only  the  larvae,  and  not  the  adults, 
are  affected.  Tower  has  described  crosses  in  which 
the  beetle  Leptinotarsa  signaticollis  was  crossed 
with  L.  undecimlineata  (Fig.  20,  A,  B).  In  the  first 
stage  (C),  the  larvae  of  these  two  beetles  are  exactly 
alike,  but  in  the  second  stage,  the  larvae  of  L.  undecim- 
lineata are  white  and  the  larvae  of  L.  signaticollis  are 
yellow ;  and  in  the  third  stage  the  undecimlineata  larvae 
are  still  white  without  stripes,  while  the  others  have 
well-developed  tergal  stripes  (B) .  When  these  species 
are  crossed  under  certain  external  conditions  the  FI 
larvae  are  yellow  and,  later,  striped.  The  beetles  that 
come  from  them  are  intermediate.  Inbred,  these 
beetles  give  three  larvae  of  the  yellow  type  to  one  of 
the  white  type. 

There  is  extensive  evidence  from  cytology,  experi- 
mental embryology,  and  regeneration,  to  show  that 
all  the  different  cells  of  the  body  receive  the  same 
hereditary  factors.  We  must  suppose,  then,  that 
the  Mendelian  factors  are  not  sorted  out,  each  to  its 
appropriate  cell,  so  that  factors  for  color  go  only  to 
pigment  cells,  factors  for  wing-shape  to  cells  of  the 
wings,  etc.,  but  that  differentiation  is  due  to  the  cumu- 
lative effect  of  regional  differences  in  the  egg  and 
embryo,  reacting  with  a  complex  factorial  background 
that  is  the  same  in  every  cell.  These  regional  peculi- 
arities of  different  parts  of  the  egg  and  embryo,  may, 


44 


TYPES    OF    MENDELIAN    HEREDITY 


like  the  age  of  the  individual,  also  be  considered  as 
influences  external  to  the  hereditary  factors  which 
affect  the  development  of  characters.  And  not  only 


FIG.  20. — Leptinotarsa  signaticollis  (above),  and  L.  undecimlineata 
(below),  with  their  full  grown  (B)  and  second  stage  (C)  larvae  to  the  right 
of  each.  (After  Tower.) 

do  regional  peculiarities  influence  characters,  but 
special  regions  are  usually  required  for  a  given  factor 
difference  to  manifest  itself,  just  as  certain  tempera- 
tures or  ages  may  be  necessary.  Thus  when  we 


TYPES  OF  MENDELIAN  HEREDITY        45 

speak  of  factors  for  eyes  or  for  legs,  we  really  mean 
factor-differences  which  can  produce  effects  only  in 
the  eye,  the  leg,  or  other  regions  of  the  body.  In 
other  cases  the  expression  of  a  factor-difference  may 
not  be  limited  to  one  region  but  may  produce  a 
different  effect  in  different  regions;  for  example,  a 
gray  white-bellied  mouse,  which  differs  from  the 
yellow  mouse  by  only  a  single  factor,  is  lighter  than 
yellow  on  the  under  side,  but  darker  on  the  upper  side. 

///.  By  the  Influence  of  Other  Factors 

Analogous  also  is  the  fact  that  certain  factor- 
differences  produce  a  visible  effect  only  when  they  are 
in  company  with  a  particular  complex  of  other  heredi- 
tary factors.  Thus,  a  fly  with  the  factors  for  ver- 
milion eyes  can  not  be  distinguished  from  one  with 
the  factors  for  pink  eyes  if  both  contain,  in  addition, 
the  factors  for  white  eyes,  for  the  factors  for  white 
allow  no  other  color  to  develop.  Again,  it  is  obvious 
that  without  the  factors  necessary  for  the  develop- 
ment of  a  given  character,  no  factors  merely  deter- 
mining special  modifications  of  that  character  can 
have  any  effect.  In  other  cases,  the  effect  of  a  given 
factor  may  not  be  entirely  suppressed,  but  greatly 
changed,  if  certain  other  factors  in  the  hereditary 
complex  are  changed.  Thus,  in  flies  which  already 
have  the  factor  for  vermilion  eyes,  the  factor  for 
purple  eyes  produces  an  eye  still  lighter  than  ver- 
milion, but  in  flies  containing  the  normal  allelomorph 
of  the  factor  for  vermilion,  the  factor  for  purple  pro- 


46  TYPES    OF    MENDELIAN    HEREDITY 

duces  an  eye  decidedly  darker  than  normal.  Such 
cases  of  interaction  of  factors,  in  which  the  effect  of 
one  factor  is  altered  by  the  action  of  another  factor, 
are  very  numerous. 

IV.  Conclusion 

It  would  have  been  indeed  strange  if  Mendelian 
factor-differences  had  not  been  found  that  require 
special  conditions — environmental,  developmental, 
or  factorial — in  order  to  produce  a  given  effect,  or 
any  effect  at  all.  For  Mendelian  factors  may  cause 
or  influence  all  sorts  of  characters — that  is,  any -or  all 
kinds  of  developmental  or  physiological  reactions; 
and  many  of  these  reactions  are  known  to  be  affected 
by  age,  temperature,  region  of  the  body,  and  so  forth. 
The  facts  given  above  are  in  no  possible  sense  sub- 
versive to  Mendelian  principles.  On  the  contrary 
they  illustrate  to  great  advantage  the  previously 
given  interpretation  of  all  hereditary  characters— 
namely,  that  every  character  is  the  realized  result  of 
the  reaction  of  hereditary  factors  with  each  other 
and  with  their  environment.  Failure  to  understand 
this  viewpoint  has  led  to  some  futile  criticism  by  the 
opponents  of  the  modern  Mendelian  interpretation 
in  terms  of  unit  factors.  This  criticism  is  as  pointless 
as  it  would  be  to  criticize  the  atomic  theory  on  the 
ground  that  oxygen  does  not,  under  all  conditions, 
and  in  all  its  compounds,  give  rise  to  substances  with 
the  same  properties. 

The  validity  of  the  unit  factor  conception  rests 


TYPES    OF    MENDELIAN    HEREDITY  47 

upon  the  fact  that  whenever  (as  often  happens)  all 
other  conditions,  external  and  internal,  that  modify 
characters  remain  constant,  clear-cut  ratios  are  ob- 
tained which  can  be  explained  only  as  due  to  segre- 
gation, in  definite  ways,  of  particular  hereditary 
factors  that  perpetuate  themselves  unchanged  from 
generation  to  generation.  The  validity  of  the  fac- 
torial hypothesis  may  also  be  proved  under  circum- 
stances not  so  well  controlled,  however.  In  cases 
where,  on  the  factorial  hypothesis,  a  certain  factor 
is  expected  to  be  present  in  an  individual,  then, 
even  if  the  individual  fails  to  develop  the  character 
commonly  taken  as  indicative  of  the  factor,  the  actual 
presence  of  the  factor  may  be  demonstrated  by  breed- 
ing tests.  For  if,  in  subsequent  generations,  cir- 
cumstances— genetic  or  environmental — are  provided, 
like  those  in  which  the  character  previously  appeared, 
it  will  again  show  itself.  Flies  of  the  race  with  ab- 
normal abdomen,  if  raised  in  a  dry  bottle,  appear 
perfectly  normal,  but  the  presence  within  them  of  the 
factor  for  abnormal  may  be  demonstrated  by  rear- 
ing their  offspring  in  a  wet  bottle.  Again,  the  factor 
for  pink  eyes  may  be  carried  by  a  race  with  white 
eyes,  and  although  pink  does  not  show  in  the  white- 
eyed  race,  its  presence  there  may  then  be  demon- 
strated by  crosses  of  these  flies  with  flies  that  are  not 
white.  Cases  like  these  could  be  multiplied  over  and 
over  again. 


CHAPTER  III 
LINKAGE 

If  two  factors  lie  in  the  same  member  of  a  chromo- 
some pair  we  should  expect  them  always  to  be  found 
together  in  successive  generations  of  a  cross  unless  an 
interchange  can  take  place  between  such  a  chromo- 
some and  the  homologous  chromosome  derived  from 
the  other  parent. 

Whenever  the  two  factors  remain  together  in  the 
same  chromosome  there  will  be  formed  equal  numbers 
of  gametes  containing  the  two  factors  and  of  gametes 
containing  the  normal  allelomorphs  of  the  two 
factors.  But  if  pieces  of  homologous  chromosomes 
are  interchanged,  then  some  of  the  gametes  will  con- 
tain one  of  the  factors  in  question,  and  an  equal 
number  will  contain  the  other  factor.  The  process 
of  interchange  between  chromosomes  is  called  cross- 
ing over;  the  tendency  of  factors  to  stay  together  is 
called  linkage. 

An  example  may  make  clearer  this  process  of  cross- 
ing over.  The  factor  for  black  body  color  and  that 
for  vestigial  wings  both  lie  in  the  second  pair  of  chro- 
mosomes. If  a  black  vestigial  fly  is  crossed  to  a 
wild  fly  (gray,  long  wings)  (Fig.  21)  the  offspring  are 
gray  with  long  wings.  These  Fi  flies  have  one  chro- 
mosome containing  both  the  factor  for  black  and  the 
factor  for  vestigial,  and  a  homologous  chromosome 

'       48 


LINKAGE 


49 


with  the  normal  allelomorphs  of  these  factors.     After 
maturation  one  or  the  other  of  these  chromosomes 


Or«y  long 


GHAT  VESTIGIAL 
8.S 


FIG.  21. — Diagram  to  illustrate  non-crossing  over  in  the  male  and 
crossing  over  in  the  female  in  a  cross  between  black  vestigial  by  gray  long 
(coupling  experiment).  The  FI  male  was  backcrossed  to  black  vestigial 
females  with  the  results  shown  to  the  left;  and  the  FI  female  was  back- 
crossed  to  black  vestigial  males,  with  the  results  shown  to  the  right. 

will  be  left  in  each  egg  and  each  sperm.     The  gametes 
will  consequently  contain  the  same  combinations  of 


50  LINKAGE 

factors  as  were  present  in  PI  unless  an  interchange 
has  taken  place  between  the  two  chromosomes.  The 
best  way  to  find  out  whether  such  an  interchange 
has  taken  place  is  to  mate  the  Fi  males  and  females 
to  the  double  recessive  type,  black  vestigial,  because 
black  and  vestigial  being  recessive  factors  will  not 
obscure  the  factors  that  are  carried  by  the  gametes 
of  the  Fi  to  be  tested.  When  the  FI  male  is  back- 
crossed  to  a  black  vestigial  female,  Fig.  21  (to  the 
left),  only  two  classes  of  offspring  are  produced. 
Half  of  the  flies  are  black  vestigial  and  half  are  gray 
long.  This  must  mean  that  there  has  been  no  cross- 
ing over  in  the  hybrid  Fi  male;  for  he  produces  only 
two  kinds  of  gametes  and  these  are  of  the  kind  that 
combined  to  produce  him.  In  other  words,  the 
chromosomes  received  from  his  parents  have  remained 
intact. 

If  we  test  the  FI  female  in  the  same  way,  Fig.  21 
(to  the  right),  the  result  is  different.  When  such  a 
female  is  bred  to  the  double  recessive  male,  black 
vestigial,  four  kinds  of  offspring  result,  as  follows: 

Non-crossovers  Crossovers 

Black,  vestigial          Gray,  long  Black,  long  Gray,  vestigial 

41.5  per  cent.    41.5  per  cent.      8.5  per  cent.          8.5  per  cent. 

83  per  cent.  17  per  cent. 

Of  these  four  classes  the  first  two  correspond  to  the 
combinations  which  the  Fi  received  from  its  parents, 
namely,  black  vestigial  and  gray  long;  the  other  two 
are  classes  that  would  be  expected  if  crossing  over  had 


LINKAGE 


51 


taken  place  between  black  and  vestigial  in  the  pair 
of  homologous  chromosomes.     The  numerical  results 


OR1T  VESTIGIAL 


FIG.  22. — Diagram,  like  that  of  Fig.  21,  to  illustrate  non-crossing  over 
in  the  male  and  crossing  over  in  the  female  when  gray  vestigial  is  mated 
to  gray  long  (repulsion  experiment).  The  percentage  of  crossovers  here 
is  the  same  as  in  Fig.  21  showing  that  the  same  percentage  results  irre- 
spective of  how  the  factors  enter. 

show  that  this  crossing  over  takes  place  in  about 
17  per  cent,  of  cases.     In  other  words,  the  chances  are 


52  LINKAGE 

about  five  to  one  that  the  combination  that  went  in 
holds  together. 

It  is  also  instructive  to  repeat  the  cross  in  such  a 
way  that  the  two  mutant  factors,  black  and  vestigial, 
enter  from  different  sides,  i.e. ,  one  parent  contributes 
black  and  the  other  vestigial.  As  shown  in  the  next 
diagram  (Fig.  22),  each  parent  carries  in  its  chromo- 
some one  mutant  factor  and  the  normal  allelomorph 
of  the  other. 

If  the  Fi  males  are  backcrossed  to  black  vestigial 
females  only  two  classes  result,  viz.,  black  long  and 
gray  vestigial,  Fig.  22  (to  the  left) .  These  are  the 
combinations  that  entered ;  hence  no  crossing  over  has 
taken  place  in  the  FI  males.  We  see  that  here  the 
linkage  is  not  due  to  some  affinity  between  the  factors 
black  and  vestigial,  per  se,  for  in  this  cross  they  always 
enter  different  gametes  as  surely  as  they  stayed 
together  before.  The  reason  for  this  difference  in 
result  is  that  in  this  cross  they  came  from  different 
parents  and  must  have  been  in  opposite  chromosomes, 
whereas  in  the  previous  cross  they  were  in  the  same 
chromosome. 

If  we  test  the  FI  females  by  mating  to  black  ves- 
tigial males,  four  classes  result,  viz., 

Non-crossovers  Crossovers 

Black,  long          Gray,  vestigial        Black,  vestigial  Gray,  long 

41.5  per  cent.    41.5  per  cent.      8.5  per  cent.        8.5  per  cent. 

83  per  cent.  17  per  cent. 

Crossing  over  has  taken  place  in  the  FI  females, 
and  the  numerical  results  show  that  this  happens  in 


LINKAGE  53 

17  per  cent,  of  cases.  Here  too  we  see  that  now  the 
factors  tend  to  separate,  whereas  in  the  case  of  the 
other  FI  female  they  tended  to  stay  together,  since 
they  lay  in  the  same  chromosome.  In  the  present 
case,  when  the  chromosomes  interchange,  the  factors 
are  brought  together,  and  so  the  crossover  classes 
are  just  the  opposite  in  the  two  cases,  as  also  are  the 
non-crossover  classes.  Yet  there  is  the  same  amount 
of  crossing  over  shown  in  both  crosses,  so  that  the 
frequency  of  the  double  recessives  and  double  domi- 
nants in  the  first  cross  is  exactly  equal  to  the  fre- 
quency of  the  single  recessive  and  single  dominants  in 
the  last  cross.  Which  classes  shall  have  the  high 
frequency  and  which  the  low  does  not  depend  on  the 
nature  of  the  factors  themselves,  therefore,  but  on 
which  ones  come  from  the  same  parent,  i.e.,  lay  in  the 
same  chromosome  at  first,  and  wThich  lay  in  opposite 
chromosomes.  The  amount  of  crossing  over  is  seen 
to  be  independent  of  the  way  in  which  the  factors  enter 
an  individual.  Hence  it  is  fair  to  infer  that  the 
process  is  not  peculiar  in  any  way  to  hybrids,  but 
takes  place  in  the  same  way  and  to  the  same  extent 
in  gametogenesis  in  pure  homozygous  stocks.  This 
is  also  indicated  by  the  fact,  later  to  be  discussed, 
that  when  several  different  allelomorphs  of  a  factor 
may  occur,  all  give  the  same  per  cent,  of  crossing 
over  with  other  factors. 

Many  other  combinations,  involving  a  large  num- 
ber of  different  characters  in  the  second  group,  have 
been  studied  and  give  consistent  results.  There  is 
never  any  crossing  over  in  the  male;  and,  in  the  fe- 


54  LINKAGE 

male,  the  amount  of  crossing  over  is  different  for 
different  factor  combinations  but,  for  any  given  com- 
bination, it  is  not  altered  by  the  way  in  which  the 
factors  entered  the  cross,  and  is,  ordinarily,1  constant. 

Tests  like  the  preceding  ones  for  the  second  group 
have  been  carried  out  for  the  third  group,  and  give 
the  same  kind  of  results.  There  is  crossing  over  in 
the  female  and  no  crossing  over  in  the  male. 

At  present  only  two  members  of  the  fourth  group 
are  known,  and  the  phenomena  of  linkage  have  not 
yet  been  studied  in  detail,  but  it  is  probable  that 
there  is  no  crossing  over  in  one  sex. 

In  the  first  group  (sex  linked  characters),  a  very 
large  amount  of  data  has  been  collected.  Here  again 
there  is  abundant  evidence  to  show  that  crossing 
over  takes  place  in  the  female,  but  not  in  the  male. 
The  curious  fact  also  comes  to  light  that  no  mutations 
have  been  discovered  in  the  Y  chromosome,  nor  does 
it  contain  any  factors  dominant  to  any  known 
mutant  or  normal  factors  in  its  mate,  the  X  chromo- 
some. Since  the  linkage  of  a  considerable  number 
of  factors  in  the  X  chromosome  has  been  studied  in 
detail  the  evidence  from  this  source  best  serves  to 
illustrate  cases  where  the  linkage  is  strong,  where  it 
is  moderate,  and  where  it  is  weak. 

The  body  color  called  yellow  and  the  eye  color 
white  have  been  used  in  many  experiments.  If  a 
yellow  white  female  is  mated  to  a  wild  male  (gray 
red)  (Fig.  23),  the  daughters  are  gray  with  red  eyes 
(like  the  fathers),  but  the  sons  are  yellow  white  like 

1  Subject  to  certain  variations  which  will  be  noted  later. 


YELLOW  WMITt  9 


GREY   REDC? 


YELLOW  WHITE 


GREY    RED 


YELLOW   RED  GREY  WHITE 

FIG.  23. — Diagram  illustrating  the  inheritance  of  two  pairs  of  sex 
linked  characters,  viz.,  yellow  white  and  gray  red.  In  F2  the  males  and 
the  females  give  the  same  classes. 


56  LINKAGE 

the  mother.  The  explanation  of  this  result  is  obvious ; 
for  the  son  gets  his  single  X  chromosome  from  his 
mother,  and  should  therefore  have  the  characters 
that  go  with  this  chromosome.  His  Y  chromosome, 
derived  from  the  father,  does  not  influence  the  result 
at  all.  The  daughters,  however,  get  one  X  chromo- 
some from  the  mother  (yellow  white)  and  the  other 
from  the  father  (gray  red).  The  factors  for  gray 
and  red  dominating  give  gray  red  daughters. 

The  composition  of  these  Fi  females  can  be  tested 
by  breeding  to  the  double  recessive  male  (yellow 
white)  since  this  does  not  carry  any  dominant  factors 
which  will  obscure  what  factors  are  received  by  the 
F2  females  from  their  mothers.  But  the  Fi  males 
are  themselves  yellow  white,  so  that  the  FI  females 
may  be  mated  to  their  brothers.  In  fact,  the  out- 
come is  the  same,  whether  a  yellow  white  male  from 
stock  or  a  yellow  white  Fi  brother  is  bred  to  the  FI 
female.  The  F2  offspring  of  such  crosses  give  the 
following  classes  and  ratios: 

Non-crossovers  Crossovers 

Yellow  white  Gray  red  Yellow  red  Gray  white 

49.5  per  cent.      49.5  per  cent.   0.5  per  cent.        0.5  per  cent. 

99  per  cent.  1  per  cent. 

This  F2  result  reveals  the  kinds  of  eggs  produced  by 
the  FI  female  (since  a  double  recessive  father  was 
used).  Crossing  over  takes  place  between  yellow 
and  white  in  only  1  per  cent,  of  cases. 

There  is  no  way  of  testing  linkage  in  the  Fi  male, 
which  is  like  a  homozygous  individual  so  far  as  the  re- 


LINKAGE  57 

suit  is  concerned,  as  his  Y  chromosome  does  not 
contain  any  factors  dominant  to  yellow  and  white, 
even  though  it  came  from  the  gray  red  male. 

The  reciprocal  cross  also  offers  certain  points  of 
interest.  When  a  gray  red  female  is  mated  to  a 
yellow  white  male  both  sons  and  daughters  are  gray 
red.  The  daughters  get  a  gray  red  chromosome 
from  the  mother  and  these  factors  dominate  the 
factors  derived  from  the  father.  The  sons  (Fi)  get 
their  single  X  chromosome  from  their  mother  and 
show  her  colors  (gray  and  red). 

If  these  gray  red  Fi  females  are  back  crossed 
to  a  yellow  white  male  they  give  the  same  numerical 
result  that  this  test  gave  in  the  reciprocal  cross,  viz., 
four  classes  of  offspring  with  1  per  cent,  of  crossing 
over. 

The  FI  males  behave  in  all  crosses  exactly  as  do 
wild  males,  which  is  to  be  expected,  since  their  single 
X  chromosome  is  derived  from  the  wild  type  mother. 

It  will  not  be  necessary  to  consider  in  detail  the 
same  cross  when  the  two  factors  enter  from  different 
parents;  they  will  now  keep  apart  exactly  to  the 
same  degree  that  they  kept  together  before.  This 
is  illustrated  for  the  backcross  as  follows: 

Non-crossovers  Crossovers 

Yellow  red  Gray  white         Yellow  white  Gray  red 

49.5  per  cent.      49.5  per  cent.  0.5  per  cent.        0.5  per  cent. 

99  per  cent.  1  per  cent. 

As  pointed  out  in  the  discussion  of  the  black  vestigial 
cross,  this  fact  is  very  important,  for  it  serves  to 


58  LINKAGE 

show  in  a  most  striking  way  that  in  the  previous  ex- 
periment with  yellow  and  white,  these  factors  hold 
together  so  strongly  from  generation  to  generation, 
not  because  of  any  innate  relation  between  these 
characters,  but  simply  because  they  started  together 
in  the  same  chromosome. 

In  the  case  of  yellow  and  white  just  given  the 
linkage  between  the  two  factors  is  very  strong  in  the 
sense  just  defined,  that  is,  they  tend  in  a  high  degree 
to  preserve  whichever  combination  they  have.  Other 
factors  show  a  different  strength  of  linkage.  For 
example,  if  a  female  with  white  eyes  and  miniature 
wings  is  bred  to  a  wild  male,  and  then  the  FI  females 
(red,  long)  are  backcrossed  to  white  miniature  males 
they  will  give  the  following  classes  of  offspring. 

Non-crossovers  Crossovers 

White  miniature        Red  long  White  long         Red  miniature 

33.5  per  cent.   33.5  per  cent.         16.5  per  cent.  16.5  per  cent. 

67  per  cent.  33  per  cent. 

The  two  large  classes,  white  miniature  and  red  long, 
correspond  to  the  combinations  that  entered.  The 
two  smaller  classes  are  the  crossover  combinations. 
Crossing  over,  therefore,  takes  place  in  33  per  cent, 
of  cases. 

Another  combination  gives  a  still  greater  amount 
of  crossing  over :  the  linkage  may  be  said  to  be  weaker. 
If  a  white  eyed  female  is  bred  to  a  bar  male  (bar  is 
a  dominant  mutation),  and  if  the  FI  females  (red 
bar  eyed)  are  bred  to  the  double  recessive  (white 
round  eyed)  sons,  the  following  classes  appear: 


LINKAGE  59 

Non-crossovers  Crossovers 

White  round  Red  bar  White  bar  Red  round 

28  per  cent.  28  per  cent.      22  per  cent.  22  per  cent. 

56  per  cent.  44  per  cent. 

Here  a  large  amount  of  crossing  over  appears,  about 
44  per  cent.  In  fact,  so  freely  do  the  factors  inter- 
change that  without  sufficiently  large  and  accurate 
numbers  the  linkage  might  entirely  escape  detection. 

THE  MECHANISM  OF  CROSSING  OVER 

If  it  be  admitted  that  the  Mendelian  factors  are 
carried  by  chromosomes  it  can  not  be  denied  that 
interchange  between  homologous  chromosomes  must 
occur,  for  sex  linked  factors  cross  over  from  each 
other,  and  yet  are  known  to  be  in  the  same  pair  of 
chromosomes,  since  they  all  follow  the  X  chromo- 
some in  its  distribution.  The  evidence  allows  for  no 
other  interpretation.  But  why  should  crossing  over 
take  place  so  rarely  between  certain  factors  and  so 
often  between  others?  We  can  make  use  here  of 
certain  information  in  regard  to  the  chromosomes 
that  gives  a  very  simple  answer  to  the  question.  In 
the  early  germ  cells,  before  the  maturation  period 
begins,  the  chromosomes  appear  to  be  scattered  in 
the  nuclei,  and  the  homologous  chromosomes  in 
many  cases  show  no  tendency  to  lie  together,  although 
in  some  animals,  e.g.  in  many  flies,  the  members  of  a 
pair  are  often  found  side  by  side.  In  this  early  period 
the  germ  cells  divide  as  do  other  cells  and  thereby 
increase  in  numbers.  But  at  the  termination  of  this 


60 


LINKAGE 


period,  the  homologous  chromosomes  unite  in  pairs. 
There  has  been  much  controversy  as  to  how  this 
union  takes  place,  but  in  some  cases  at  least,  the 
uniting  chromosomes  twist  around  each  other  as 
they  come  together.  This  is  illustrated  to  the  left 
in  Fig.  24.  As  a  consequence,  parts  of  one  chromo- 


n 


FIG.  24. — Diagram  to  represent  crossing  over.  At  the  level  where  the 
black  and  the  white  rod  cross  in  ..4,  they  fuse  and  unite  as  shown  in  D. 
The  details  of  the  crossing  over  are  shown  in  B  and  C. 

some  will  come  to  lie  now  on  one,  now  on  the  other 
side  of  the  mate.  If  when  the  twisted  chromosomes 
separate,  the  parts  on  the  same  side  go  to  the  same 
pole  the  end  result  will  be  that  shown  to  the  right 
in  Fig.  24.  Each  chromosome  has  interchanged  a 
part  with  its  mate.  This  process  has  been  called 
crossing  over.  It  is,  of  course,  also  possible  that  the 
twisted  chromosomes  do  not  break  and  reunite  where 


LINKAGE  61 

they  cross,  and  if  they  do  not  then  when  they  begin 
to  separate  they  simply  pull  apart  irrespective  of  the 
side  on  which  they  lie.  When  this  occurs  each 
chromosome  remains  intact  and  no  crossing  over 
takes  place. 

Later  some  of  the  evidence  on  which  the  above 
statements  rest  will  be  examined  more  critically. 
For  the  present  it  need  only  be  pointed  out  that 
such  a  crossing  over  of  parts  of  the  chromosomes 
would  supply  the  necessary  mechanism  to  account 
for  interchange.  The  chance  that  such  a  process  of 
crossing  over  will  occur  between  any  two  given  points 
on  the  chromosome  should  obviously  be  greater, 
the  greater  the  distance  between  those  points.  If 
then  the  Mendelian  factors  lie  along  the  chromo- 
somes, the  amount  of  crossing  over  between  any  two 
of  them  will  depend  on  their  distance  apart.  Should 
two  points  lie  near  together  a  crossover  will  only 
rarely  occur  between  them;  if  they  lie  further  apart 
the  chance  of  such  a  crossover  taking  place  at  some 
point  between  them  will  be  greater.  From  this 
point  of  view  the  percentage  of  crossing  over  is  an 
expression  of  the  " distance"  of  the  factors  from  each 
other. 

In  this  way  the  diagram  shown  in  the  frontispiece 
has  been  constructed.  Not  only  can  all  the  facts 
of  linkage  so  far  studied  be  explained  on  this  basis, 
but,  as  will  now  be  shown,  certain  further  results  can 
be  predicted.  This  is  illustrated  in  what  may  be 
called  a  three-point  experiment,  i.e.,  an  experiment 
in  which  three  pairs  of  factors  are  involved. 


62  LINKAGE 

•» 

The  three  factors  already  studied,  namely,  white, 
miniature,  and  bar,  furnish  an  excellent  illustration. 
If  we  represent  the  percentages  of  crossing  over  as 
relative  distances  along  the  chromosome  the  three 
points  will  lie  as  shown  in  Fig.  25. 

If  crossing  over  takes  place  between  white  and 
miniature  and  between  miniature  and  bar,  then  it 


FIG.  25. — Diagram  to  illustrate  double  crossing  over.  The  white  and 
the  black  rods  (a)  twist  and  cross  at  two  points.  Where  they  cross  they 
are  represented  as  uniting  (shown  in  c).  That  an  interchange  of  pieces 
has  taken  place  between  W  and  Br  is  demonstrated  by  the  factor  M 
having  gone  over  to  the  other  chromosome. 

might  be  expected  sometimes  to  take  place  in  both 
regions  at  once,  as  shown  in  Fig.  25,  6.  The  result  here 
would  be  to  produce  two  chromosomes  like  those 
showrn  in  the  lower  figure.  The  combinations  of 
factors  which  these  two  chromosomes  resulting  from 
double  crossing  over  would  contain,  are  white  long 
bar  and  red  miniature  round.  Since  these  two  classes 


LINKAGE  63 

of  gametes  are  actually  produced,  the  results  of  the 
experiment  fulfil  the  theoretical  expectation. 

There  is  a  corollary  of  importance  to  this  conclu- 
sion. When  a  cross  is  made  that  involves  only  white 
and  bar,  the  double  crossing  over,  that  can  be  de- 
tected only  when  an  intermediate  point  is  followed, 
must  still  be  supposed  to  take  place.  Whenever  it 
does  take  place  white  bar  flies  and  red  round  flies 
result.  These  will  be  added  to  the  non-crossover 
classes  since  they  have  the  same  external  character- 
istics. Consequently,  the  non-crossover  classes  will 
be  increased  and  the  crossover  classes  decreased. 
In  fact,  the  sum  of  the  two  crossover  percentages 
33  and  22  (55)  is  much  greater  than  the  apparent 
amount  (44)  of  crossing  over  when  only  white  and 
bar  are  involved.  Here  then  we  have  an  explanation 
of  why  long  distances  taken  as  a  whole  give  too  little 
crossing  over,  as  compared  with  the  same  distances 
taken  section  by  section.  The  lowered  percentage 
is  an  actual  mathematical  necessity  owing  to  the 
occurrence  of  double  crossing  over. 

In  the  case  of  double  crossing  over  the  two  points 
of  crossing  over  can  not  be  near  together  unless  the 
chromosomes  are  tightly  twisted.  Consequently, 
when  crossing  over  occurs  at  any  point  the  region  on 
each  side  should  be  protected  from  further  crossing 
over.  That  this  actually  happens  may  now  be  dem- 
onstrated. For  example,  from  vermilion  to  sable  is 
10  units,  and  from  sable  to  bar  is  14  units  more  (as 
seen  in  the  frontispiece).  If  crossing  over  occurs 
between  vermilion  and  sable  the  region  between 


64  LINKAGE 

sable  and  bar  should  be  somewhat  protected  from 
crossing  over.  The  usual  amount  of  crossing  over 
between  sable  and  bar  is  14  per  cent.,  but  in  those 
cases  in  which  crossing  over  between  vermilion  and 
sable  occurs,  this  value  becomes  reduced  to  somewhat 
less  than  4  per  cent.  In  this  same  fashion  a  region 
just  to  the  left  of  sable  is  protected,  but  this  protec- 
tion decreases  with  the  distance  from  the  vermilion 
sable  region.  The  fact  that  one  crossing  over  makes 
less  likely  another  crossing  over  in  a  nearby  region, 
or  in  a  sense  interferes  with  a  second  crossing  over 
nearby,  is  called  interference.  As  has  been  shown, 
interference  decreases  with  increase  of  distance.1 

In  the  construction  of  the  chromosome  maps  shown 
in  the  frontispiece  the  distance  taken  as  a  unit  is 
that  within  which  1  per  cent,  of  crossing  over  will 
occur.  Thus,  yellow  and  white  are  placed  one  unit 
apart,  since  there  is  1  per  cent,  of  crossing  over  be- 
tween yellow  and  white.  White  and  bifid  give  5  per 
cent,  of  crossing  over,  hence  they  are  placed  five  units 
apart;  and  since  yeUow  and  bifid  give  6  per  cent., 
bifid  must  be  on  the  other  side  of  white  from  yellow. 
In  a  similar  way  the  relative  positions  of  the  other 
factors  have  been  plotted,  the  position  of  any  factor 
on  the  map  being  determined,  as  far  as  possible,  by 

1  If  it  should  be  found  that  crossing  over  takes  place  at  a  stage  when 
the  chromosomes  actually  are  tightly  twisted,  there  is  no  evident  mech- 
anism which  would  tend  to  prevent  crossing  over  from  taking  place  at 
two  points  near  together,  unless  in  this  case  we  should  suppose  that 
crossing  over  results  from  a  breaking  of  the  threads  at  some  point  due  to 
the  strain  of  very  tight  twisting,  and  that  a  break  at  one  point  relieves 
the  strain  in  the  vicinity,  thus  tending  to  prevent  another  crossing  over 
nearby. 


LINKAGE  65 

the  per  cent,  of  crossing  over  between  it  and  the  factor 
nearest  to  it.  In  general,  it  may  be  said  that  the 
number  of  units  of  distance  on  the  map  between  any 
two  factors  (A  and  C),  will  equal  the  per  cent,  of 
crossing  over  that  will  actually  be  observed  between 
them  in  an  experiment  involving  these  two  pairs  of 
factors,  even  although  their  distance  on  the  map  may 
not  have  been  obtained  directly  from  their  linkage 
with  each  other,  their  positions  having,  instead,  been 
determined  by  their  linkage  with  other  factors.  On 
account  of  double  crossing  over,  however,  this  would 
not  be  expected  to  hold  for  very  long  distances;  and, 
as  has  been  explained,  we  do  actually  find  that,  if 
long  distances  are  involved,  the  distance  between  A 
and  C  determined  as  on  the  map,  by  adding  the  inter- 
mediate distances  A-B  and  B-C,  is  longer  than  the 
distance  AC  as  directly  determined  in  an  experiment 
involving  only  these  two  pairs  of  factors.  It  never- 
theless remains  true  that,  given  the  distance  between 
any  two  factors  on  the  map,  the  per  cent,  of  crossing 
over  between  them  can  always  be  calculated  from  this 
distance  (since  the  amount  of  discrepancy  due  to 
double  crossing  over  also  depends  on  the  distance); 
this  shows  that  the  amount  of  crossing  over  between 
them  is  an  expression  of  their  position  in  a  linear 
series.  This  striking  fact,  that  the  mathematical 
relations  between  the  various  linkage  values  conforms 
to  a  linear  series,  is  a  strong  argument  that  the  factors 
are  actually  arranged  in  line  in  the  chromosomes. 
If  the  relations  between  the  various  linkage  values 
were  not  determined  by  some  linear  relation  of  the 


66  LINKAGE 

factors  but  were  of  a  random  sort,  these  relations 
could  not  be  calculated  from  a  linear  map. 

As  a  concrete  illustration  of  the  way  in  which  a 
group  of  factors  behaves  as  a  linear  series,  attention 
may  be  called  to  the  manner  of  distribution  of  the 
factors  among  the  germ  cells  of  a  female  heterozygous 
for  a  large  number  of  factors  in  the  same  pair  of 
chromosomes.  Let  us  write  the  factors  derived  from 
one  parent,  i.e.,  those  in  one  of  the  chromosomes,  on 
one  line  (see  formula  p.  67),  in  the  order  which  they 
have  on  the  map  (see  frontispiece),  and  the  allelo- 
morphic  factors  derived  from  the  other  parent,  i.e., 
those  in  the  homologous  chromosome,  in  correspond- 
ing positions  on  the  line  below.  Then  in  such  a  case 
the  mature  eggs  contain  either  all  of  the  factors 
represented  on  one  line  and  none  of  those  on  the  other, 
or  they  contain  all  of  the  factors  present  in  one  section 
of  the  line,  and  all  of  the  factors  present  in  the  re- 
maining section  of  the  other  line.  In  other  words, 
the  factors  obviously  stick  together  in  sections  ac- 
cording to  their  position  in  the  linear  series.  When 
double  crossing  over  occurs  the  line  is  broken  in  two 
places,  but  even  here  whole  sections  remain  intact. 

The  above  facts  may  be  illustrated  by  an  actual 
case.  The  first  formula  shows  the  composition  of 
a  hybrid  female  which  has  received  from  her  mother 
the  mutant  factors:  yellow,  white,  abnormal,  bifid, 
vermilion,  miniature,  sable,  rudimentary,  and  forked, 
and  from  her  father  the  normal  allelomorphs  of  these 
factors,  together  with  the  dominant  mutant  factor, 
bar. 


LINKAGE  67 

ywabjvms     rfb' 
YWABi    VMS    RFB' 

A  number  of  females  of  this  type  have  been  made 
up  by  Muller.  The  next  formula  shows  the  kinds  of 
eggs  that  were  produced  by  one  of  these  females  and 
the  numbers  of  each  kind  that  were  produced. 

Non-crossovers : 

y  w  a  bj      v  m  s     r   f  b'-6. 
Y  W  A  Bi     VMS     R  F  B'-  8. 

Single  crossovers: 

YWabi  vms  rfb' -2. 

YWABj  vms  rf  b'-2. 

y  w  a  b;  VMS  R  F  B'-  2. 

YWABj  Vms  r  f  b'-l. 

YWABi  VMS  r  f  b'-l. 

ywab;  vms  RF  B'- 1. 

Double  crossover: 
y  w  a  bs     VMS     RFb'-l. 

Counts  of  over  600  offspring  from  females  of  the 
same  type  have  given  similar  results.  The  character- 
istic method  of  interchange  here  demonstrated  may 
perhaps  be  better  realized  by  contrasting  the  com- 
binations just  given  with  the  following,  which  illus- 
trate types  of  eggs  found  not  to  be  produced  by  such 
females  • 

yWaBi    VmS     rfB' 
Y  W  a  bi     Vms     RfB' 

It  is  not  supposed,  however,  that  the  per  cent,  of 


68  LINKAGE 

crossing  over  represents  precisely  the  distance  between 
the  factors,  for  it  may  be  that  crossing  over  is  more 
likely  to  take  place  in  one  region  of  the  chromosome 
than  in  another.  In  that  case  the  distances  between 
factors  in  this  region  calculated  from  the  amount  of 
crossing  over  between  them,  would  be  relatively 
greater  than  the  actual  distance.  It  is  supposed, 
however,  that  at  least  the  order  of  the  factors  in  the 
diagram  represents  their  real  order.  Sturtevant 
has  found  definite  factors  which  alter  the  amount 
of  crossing  over  in  the  chromosomes,  and  these  factors 
actually  do  affect  the  amount  of  crossing  over  differ- 
ently in  the  different  regions.  A  map  of  the  chromo- 
somes based  upon  the  per  cent,  of  crossing  over  when 
these  factors  are  present  would  show  different  rela- 
tive distances  between  the  loci  than  those  calculated 
from  the  normal  linkage  values.  It  is  to  be  noted, 
however,  that  even  in  these  diagrams,  the  order  of 
the  factors  remains  unchanged.  One  of  the  factors 
lies  in  the  second  chromosome  and  lowers  the  amount 
of  crossing  over  in  certain  regions  of  this  chromosome ; 
the  other  lies  in  the  third  and  apparently  affects 
only  this  chromosome,  and  chiefly  the  end  of  this 
chromosome  in  which  it  itself  is  located.  Bridges  has 
found  that  the  percentage  of  crossing  over  in  the  sec- 
ond chromosome  is  also  lowered  with  increase  in  the 
age  of  the  female,  and  it  may  be  that  other  influences 
as  well  may  affect  the  amount  of  crossing  over. 
This  variation  in  crossing  over  is  in  no  way  preju- 
dicial to  the  conception  of  crossing  over  above  out- 
lined. Variation  in  the  amount  of  crossing  over  has 


LINKAGE  69 

also  been  found  in  other  forms  than  Drosophila,  but 
in  these  cases  the  determining  conditions  and  their 
effect  on  the  various  linkage  values  have  not  as  yet 
been  discovered. 

LINKAGE  IN  OTHER  ANIMALS  AND  IN  PLANTS 

Since  the  discovery  in  1906  of  linkage  in  sweet  peas 
many  cases  have  been  found  in  animals  and  in  plants. 
In  sweet  peas  themselves  two  groups  of  linked  factors 
are  now  known,  one  containing  three  pairs  of  factors 
and  the  other  three  or  possibly  four.  In  garden 
peas  there  are  two  pairs  of  linked  factors  and  two 
other  cases  that  are  doubtful;  in  the  primrose  there 
is  a  group  of  five  pairs  of  linked  factors;  in  the  snap- 
dragon there  is  a  group  of  three  pairs;  in  stocks  there 
is  a  group  of  three  or  probably  four  pairs.  In  animals, 
linkage,  aside  from  sex  linkage,  has  been  discovered 
in  only  one  form  besides  Drosophila,  viz.,  the  silk- 
worm, in  which  Tanaka  has  found  that  several  linked 
factors  are  present,  i.e.,  four  allelomorphs  in  one 
locus  linked  to  two  allelomorphs  in  another  locus. 
There  are,  it  is  true,  several  other  cases  in  which  the 
evidence  leads  one  to  suspect  that  linkage  occurs,  but 
these  are  too  uncertain  at  present  to  be  included  in 
the  list.  In  all  the  above  cases  the  linkage  is  "par- 
tial," that  is,  a  certain  amount  of  crossing  over  takes 
place,  at  least  in  one  sex. 

There  are  a  number  of  cases  of  sex  linkage,  which, 
being  only  a  special  case  of  linkage,  undoubtedly 
belong  in  the  same  category,  but  the  amount  of  cross- 


70 


LINKAGE 


ing  over  between  the  sex  factor  and  the  various  sex- 
linked  factors  can  not  be  calculated,  since  in  the  sex 
that  is  heterozygous  for  the  sex  factor  no  crossing 
over  has  been  observed.  Sex  linkage  has  been  found 


FIG.  26. — Black  Langshan  female  by  Barred  Plymouth  Rock  male. 
Com  pare  with  Fig.  30  (similar  cross  in  Abraxas)  for  scheme  of  inheritance, 
which  is  the  same  in  both.  Substitute  Black  for  lacticolor  and  Bar  for 
grossulariata. 

in  the  moths  Abraxas  (Figs.  30  and  31)  and  Lyman- 
tria,  in  the  fowl  (Figs.  26,  27,  28,  29)  (six  factors), 
canary,  pigeon,  Drosophila  (Figs.  9  and  10),  fish,  cat, 
man,  and  the  plant  Lychnis.  In  all,  somewhat  more 
than  fifteen  species  show  linkage. 

This  number  appears  small  in  comparison  with  the 


LINKAGE 


71 


large  number  of  species  in  which  Mendelian  inheri- 
tance has  been  discovered;  but  there  are  several  rea- 
sons why  more  cases  have  not  been  recorded.  In 
the  first  place,  the  number  of  chromosomes  is  generally 
large  compared  with  the  number  of  characters  that 


FIG.  27. — Barred  Plymouth  Rock  female  by  Langshan  male.     Compare 
similar  cross  in  Abraxas  for  scheme  of  inheritance. 

have  been  studied  in  such  a  way  that  linkage  would 
be  noticed.  Thus,  there  is  little  chance  of  finding 
two  factors  lying  in  the  same  chromosome.  Sec- 
ondly, unless  this  linkage  is  close,  it  might  easily 
escape  detection,  especially  when  the  number  of  off- 


72 


LINKAGE 


spring  recorded  is  small.     In  such  cases  the  data  are 
usually  fitted  to  the  nearest  "Mendelian"  ratio  even 


3 

FIG.  28. — Photograph  of  the  Pi  (1  and  2)  and  FI  (3  and  4)  birds  in  such 
a  cross  as  that  of  Fig.  26. 

though  discrepancies  are  apparent.  Even  in  species 
where  a  number  of  different  characters  have  been 
studied  these  are  often  recorded  in  separate  tables, 


LINKAGE 


73 


which    excludes    the    possibility    of    detecting    any 
linkage  that  is  present,  for  obviously  linkage  cannot 


3 


4 


FIG.  29. — Photograph  of  Pi  (1  and  2)  and  f\  (3  and  4)  birds  in  such  a  cross 
as  that  of  Fig.  30.     The  PI  male  is  a  standard  figure. 


be  seen  unless  at  least  two  pairs  of  factors  are  studied 
at  the  same  time.     The  steady  increase  in  the  number 


74  LINKAGE 

of  cases  of  linkage  that  is  occurring  at  the  present 
time,  when  the  importance  of  detecting  them  has 
become  apparent,  and  the  methods  for  studying 
them  have  been  worked  out,  appears  to  presage  the 
realization  of  linkage  as  a  general  phenomenon.  Its 
occurrence  in  such  widely  separated  types  is  also  a 
sign  that  it  is  a  constant  accompaniment  of  Mende- 
lian  inheritance. 

THE  REDUPLICATION  HYPOTHESIS 

Linkage  has  been  interpreted  by  Bateson  and  his 
co-workers  on  a  basis  entirely  different  from  that 
adopted  in  this  book.  These  investigators  do  not 
connect  Mendelian  factors  with  the  chromosomes  in 
any  way,  and  do  not  suppose  that  segregation  occurs 
at  the  reduction  division.  In  a  case  of  linkage  be- 
tween two  pairs  of  factors,  Aa  and  Bb,  the  doubly 
heterozygous  individual  will  have  the  formula  ABab. 
Bateson  supposes  that  in  such  an  individual  segre- 
gation takes  place  before  the  reduction  division— 
perhaps  in  early  cleavage  stages,  perhaps  after  the 
formation  of  the  gonads.  Two  cell  divisions  are 
required  for  this  segregation.  In  the  first,  A  and  a 
do  not  divide,  but  one  goes  to  each  daughter  cell, 
i.e.,  they  segregate.  B  and  b,  however,  both  divide, 
and  each  daughter  cell  receives  both  B  and  b.  The 
resulting  cells  then  have  the  formulae,  ABb  and  aBb, 
respectively.  In  other  words,  A  and  a  have  segre- 
gated, but  B  and  b  have  not.  At  the  next  division 
B  and  b  segregate,  giving  four  cells,  with  the  combina- 


LINKAGE  75 

tions  AB,  Ab,  aB,  and  ab,  respectively.  These  cells 
then  proceed  to  divide,  the  number  of  divisions  not 
being  the  same  for  each,  which  results  in  the  produc- 
tion of  more  of  some  kinds  of  cells  than  of  others. 
But  this  multiplication  must  be  assumed  to  be  a 
symmetrical  process,  since  the  observed  number  of 
AB  gametes  equals  the  number  of  ab,  and  similarly 
Ab  equals  aB.  The  whole  process  just  described 
is  known  as  "reduplication."  The  term  is  applied 
to  the  same  cases  as  those  included  under  the  name 
of  linkage. 

When  three  pairs  of  factors  are  involved  in  the  same 
"reduplication  series"  Bateson  supposed  at  one  time 
that  they  are  segregated  at  three  successive  cell 
divisions,  after  which  the  eight  resulting  cells  divide 
at  unequal  rates.  Later  Trow  suggested  for  such  a 
case  that  perhaps  only  two  segregating  divisions  occur 
at  first,  producing  the  cells  ABCc,  AbCc,  aBCc,  and 
abCc,  which  may  then  multiply  so  as  to  give  the 
proper  proportions  for  the  A  and  B  combinations. 
After  this  there  occurs  in  every  cell  a  division  which 
segregates  C  and  c.  The  resulting  cells  then  divide 
again  so  as  to  produce  the  observed  relations  be- 
tween the  C  pair  and  the  other  factors. 

The  nature  of  the  factors  themselves  in  the  differ- 
ent lines  of  cells  resulting  from  segregation  can  not 
be  supposed  to  determine  the  difference  in  the  number 
of  times  that  these  lines  divide,  because  if  an  indi- 
vidual has  received  AB  from  one  parent  and  ab  from 
the  other,  the  lines  of  cells  reduplicate  in  a  way  just 
opposite  to  that  in  an  individual  which  received  Ab 


76  LINKAGE 

from  one  parent  and  aB  from  the  other.  In  one 
individual  the  line  AB  divides  a  certain  number  of 
times  more  than  aB,  whereas  in  the  other  aB  divides 
just  that  many  times  more  than  AB.  In  other 
words,  the  number  of  times  a  line  of  cells  divides  must 
be  assumed  to  be  determined  in  some  way  by  whether 
or  not,  in  its  formation,  certain  factors  separated 
that  had  established  a  relation  with  each  other  by 
being  present  together  in  the  egg  or  sperm  from  which 
the  individual  came.  To  explain  this,  Bateson  and 
Punnett  have  suggested  that  at  the  time  of  fertiliza- 
tion there  is  established  in  the  egg  a  "polarity" 
which  determines  the  planes  of  the  segregating  divi- 
sions. But  it  seems  impossible  to  imagine  how  this 
or  any  other  mechanism  could  bring  about  the  above 
result.  On  attempting  to  follow  out  in  concrete 
detail  the  events  wrhich  must  be  assumed  to  occur  in 
any  case  of  reduplication,  we  find  that,  if  the  above 
stated  relation  is  to  hold,  then,  on  "polarity"  or  any 
other  hypothesis,  the  assumption  of  the  most  intricate 
and  improbable  relations  and  processes  is  forced  upon 
us. 

This  interpretation  of  linkage  was  originally  based 
largely  upon  the  supposed  fact  that  the  "game tic 
ratios"  (ratio  of  parental  combinations  to  new  or 
crossover  combinations  in  the  gametes)  fell  into  the 
series  1:1:1:1, 3:1:1:3,  7:1:1:7, 15:1:1 :15,  31:1:1  • 
31,  etc.  The  supposed  connection  between  this 
series  and  reduplication  is  too  involved  to  explain 
here,  and  gametic  ratios  which  do  not  fall  into  it  are 
now  definitely  known.  In  fact,  it  seems  probable  that 


LINKAGE  77 

ratios  which  do  fall  into  it  are  no  more  frequent  than 
would  be  expected  from  a  chance  distribution. 

Another  assumption  upon  which  the  reduplication 
hypothesis  is  based  is  the  old  idea  of  somatic  (pre- 
reductional)  segregation.  This  hypothesis,  once  ad- 
vocated by  Roux  and  Weismann  as  an  explanation  of 
differentiation,  is  opposed  by  a  large  body  of  experi- 
mental evidence  from  the  fields  of  regeneration  and 
experimental  embryology,  and  has  been  given  up  by 
practically  all  students  of  developmental  mechanics, 
including  Roux  himself. 

At  first  it  was  doubted  whether  more  than  two  pairs 
of  factors  could  show  reduplication  in  the  same  organ- 
ism, but  when  it  was  experimentally  proven  that  two 
pairs  were  not  the  limit,  the  scheme  was  extended. 
When  game  tic  ratios  not  falling  into  the  3,  7,  15, 
series  were  found,  the  theory  was  modified  to  permit 
other  ratios.  When  it  was  found  that  the  result 
depended  upon  the  way  in  which  the  factors  entered 
the  cross,  the  " polarity"  hypothesis  was  added. 
Some  further  extension  will  be  necessary  to  account 
for  the  phenomenon  of  "  interference "  discussed 
above. 


CHAPTER  IV 
SEX  INHERITANCE 

There  are  two  types  of  sex  inheritance  known  in 
those  species  in  which  separated  sexes  exist.  In  one 
type,  which  may  be  called  the  Drosophila  type  (XX- 
XY  type,  or,  for  short,  the  XY-type),  the  female  is 
homozygous  for  a  sex  factor,  the  male  heterozygous; 
in  the  other,  the  Abraxas  type  (the  WZ-ZZ  type,  or, 
for  short,  the  WZ  type)  the  female  is  heterozygous  for 
a  sex  factor,  the  male  homozygous.  Since  in  both 
cases  the  heterozygous  individuals  must  always  mate 
with  the  homozygous  ones  there  should  result  in  each 
succeeding  generation  equal  numbers  of  heterozygous 
and  homozygous  individuals,  and  so  the  bisexual  con- 
dition is  perpetuated  as  follows: 


so 


The  genetic  evidence  so  far  gained  has  placed  in  the 
Drosophila  type  the  following  animal   forms:  Dro- 


78 


SEX    INHERITANCE  79 

sophila,  man,  cat;  and  the  plants,  Lychnis  and  Bry- 
onia.  The  cytological  evidence  refers  to  the  same 
type  the  insect  groups  of  bugs,  flies,  beetles,  grass- 
hoppers; the  spiders,  certain  worms  (Ascaris),  echino- 
derms,  amphibia  and  mammals  (including  man). 
The  genetic  evidence  has  placed  in  the  Abraxas 
type  several  moths  and  butterflies,  and  several  birds; 
viz.,  chickens,  ducks,  and  canaries.1  Favorable 
cytological  evidence  has  been  found  only  in  the  case 
of  a  few  moths. 

In  many  cases  of  the  Drosophila  type,  in  which 
the  history  of  the  sex  chromosomes  has  been  worked 
out  cytologically,  it  has  been  found  that  in  the  male 
there  is  a  pair  of  chromosomes,  the  two  members 
of  which  are  different  in  size  or  shape.  These  are  the 
"sex  chromosomes"  and  are  designated  as  X  and  Y. 
In  many  species  of  the  Drosophila  type  the  Y  is 
slightly  smaller  than  the  X,  and  in  the  various  other 
species  of  this  type  all  gradations  in  the  relative  size 
of  the  Y  are  found,  between  this  condition  and  the 
condition  where  Y  is  completely  absent.  In  some 
related  species,  on  the  other  hand,  the  chromosomes 
which  obviously  correspond  to  X  and  Y  are  alike  in 
appearance.  It  is  not,  after  all,  the  size  difference 
usually  visible  in  the  male,  between  X  and  Y,  which 
gives  these  two  chromosomes  their  significance  in  sex 
determination,  but  rather  a  difference  in  the  factors 
they  contain.  The  size  difference  is  an  incidental 
concomitant,  or,  as  it  were,  a  token  or  label  that  is 

1  Richardson's  work  on  strawberries  suggests  that  this  plant  may  come 
under  the  Abraxas  type 


80 


SEX    INHERITANCE 


not  present  in  all  species.  In  all  these  cases  the 
female  contains  two  X  chromosomes,  the  Y  chromo- 
some being  confined  to  the  male  line. 

This  type  of  sex  determination  represents  all  eggs 
as  alike — each  containing  one  X  (after  the  polar 
bodies  have  been  extruded),  but  the  sperm  is  of 
two  kinds,  one  containing  the  X  and  the  other  Y,  or 
merely  no  X.  The  scheme  is  as  follows: 

xx  XY 


It  will  be  seen  that  all  the  spermatozoa  carrying  X 
produce  females,  while  all  those  carrying  Y  or  no  X 
produce  males. 

The  Y  chromosome,  when  present,  descends  from 
father  to  son.  It  might  seem,  therefore,  that  if  the 
Y  carried  a  sex  factor  for  maleness  the  scheme  would 
work  out  as  well  as  if  a  sex  factor  were  carried  by  the 
X  chromosome.  But  in  several  cases  there  is  no  Y 
in  the  male,  and  in  certain  cases  to  be  described  later, 
due  to  non-disjunction,  there  are  females  that  have  a 
formula  XXY  and  yet  their  sex  is  not  affected  in  any 
way  on  account  of  the  presence  of  the  supernumerary 
Y.  It  follows  that  sex  is  not  determined  by  the 
presence  or  absence  of  the  Y  chromosome  but  by  the 


SEX    INHERITANCE  81 

number  of  the  X  chromosomes  that  are  present.  In 
the  cases  that  follow,  where  sex  determination  of  the 
Drosophila  type  was  discovered  by  a  study  of  sex 
linked  inheritance,  as  well  as  in  the  above  cases,  where 
the  mechanism  was  discovered  through  cytological 
observations,  proof  that  the  male  is  heterozygous  for  a 
Mendelian  factor  for  sex  is  derived  from  the  fact  that 
he  gives  rise  to  two  kinds  of  spermatozoa — male  pro- 
ducing and  female  producing — in  equal  numbers. 
We  know  this  in  the  cases  worked  out  cytologically 
because  here  the  spermatozoa  carrying  X  must  all 
produce  females,  while  the  other  half  must  produce 
males ;  and  we  know  it,  in  the  cases  worked  out  gen- 
etically, because  here  only  half  the  spermatozoa  from 
a  male  with  a  dominant  sex  linked  character  carry 
the  dominant  factor,  and  these  all  produce  females, 
while  the  rest  produce  males.  The  female  must  con- 
tain the  same  Mendelian  sex  factor  as  is  present  in 
the  female-producing  spermatozoa  of  the  male ;  but 
the  female  must  be  homozygous  for  this  factor,  since 
any  egg,  if  fertilized  by  a  male-producing  sperma- 
tozoon, contributes  this  factor  to  the  resulting  male. 

Although  the  only  way  in  which  the  results  of  sex 
linked  inheritance  of  the  Drosophila  type  differ 
from  non-sex  linked  cases  is  the  one  above  stated, 
namely,  that  a  dominant  male  transmits  his  dominant 
sex  linked  factor  only  to  his  daughters,  nevertheless 
it  may  be  well  at  this  point  to  recall  specifically  what 
ratios  are  produced  in  consequence,  in  the  various 
types  of  crosses. 

Examples  of  sex  linked  inheritance  in  Drosophila 


82  SEX    INHERITANCE 

have  already  been  given ;  that  of  white  eyes  is  typical 
of  all  the  rest.  The  main  facts  may  be  restated 
here.  If  a  white  eyed  male  is  bred  to  a  red  eyed 
female  the  offspring  are  red  eyed  (Fig.  9).  If  these 
are  inbred  all  of  the  F2  daughters  are  red  eyed,  but 
half  of  the  sons  are  white  eyed  and  half  red  eyed. 
In  a  word,  the  grandfather  transmits  his  characters 
visibly  to  half  of  his  grandsons  but  to  none  of  his 
granddaughters. 

In  the  reciprocal  cross  (Fig.  10),  a  white  eyed 
female  bred  to  a  red  eyed  male  produces  the  criss- 
cross result  of  red  eyed  daughters  and  white  eyed 
sons.  These  give  white  and  red  eyed  males  and  fe- 
males in  equal  numbers.  On  the  assumption  that  the 
factor  for  white  eyes  is  carried  by  the  sex  chromo- 
somes the  inheritance  of  white  eyes  can  be  readily 
understood.  It  will  be  observed  that  a  female  trans- 
mits to  each  of  her  sons  one  of  her  X  chromosomes 
with  all  the  factors  contained  in  it.  Her  sons  will 
show  all  of  these  sex  linked  characters  whether  they 
be  dominant  or  recessive  since  they  receive  no  other 
X  to  dominate  those  characters  and  the  Y  contains 
no  dominant  factor.  For  example,  if  a  stock  be 
made  up  pure  for  yellow  body  color,  white  eyes,  ab- 
normal abdomen,  bifid  wings,  sable  body  color,  forked 
spines  and  bar  eyes,  and  if  a  female  of  this  stock  be 
bred  to  a  wild  male,  all  of  her  sons  will  be  yellow, 
white,  abnormal,  bifid,  sable,  forked  and  bar.  The 
daughters,  however,  will  receive  not  only  this  chro- 
mosome from  their  mother,  but  will  also  receive  a 
chromosome  from  the  wild  male  (their  father)  con- 


SEX    INHERITANCE 


83 


taining  the  normal  allelomorphs  of  all  these  factors. 
In  the  case  of  all  the  factor-pairs,  except  abnormal 
and  bar,  the  normal  allelomorph  dominates.  There- 
fore, the  females  will  appear  normal  for  all  characters 
except  abnormal  and  bar,  which  are  dominant. 

In  the  cat,  Doncaster  has  discovered  a  sex  linked 
factor  affecting  the  coat  color.  In  man  several  char- 
acters, such  as  color  blindness,  haemophilia,  and 
others  less  certainly  identified  have  been  found  to 
follow  the  same  scheme. 

A  comparison  of  sex  linkage  in  Abraxas  with  that 
in  Drosophila  shows  that  the  mode  of  inheritance  of 
sex  linked  characters  is  identical  in  these  two  cases, 
but  the  sex  relations  are  exactly  reversed.  In  the 
Abraxas  type  sex  linked  inheritance  takes  place  in 
accord  with  the  plan  that  the  female  is  heterozygous 
in  sex  production.  If  the  chromosome  that  carries 
this  sex  differentiator  is  called  Z,  and  its  mate  in 
the  female  W,  the  formula  for  the  male  would  be  ZZ 
and  that  for  the  female  WZ.  The  scheme  follows: 


wz  iz 

Inheritance  in  Abraxas  is  illustrated  in  the  follow- 
ing diagrams  (Figs.  30  and  31),  in  which  the  common 


84 


SEX    INHERITANCE 


wild  type  A.  grossulariata  is  crossed  to  the  rare  mu- 
tant type  A.  lacticolor. 


LACTICOLOR  $ 


CROSSULARIATA    rf 


FIG.  30. — Abraxas  lacticolor  female  by  A.  grossulariata  male.  The  sex 
chromosomes  are  represented  by  the  circles  in  the  center  of  the  diagram, 
and  the  letters  contained  in  them  stand  for  the  factors  that  each  carries. 
The  W  chromosome,  confined  to  the  female  line,  is  represented  without 
either  G  or  L;  for  it,  Like  the  Y  chromosome  in  Drosophila,  carries  no  sex 
linked  factors. 


SEX    INHERITANCE 


85 


In  the  first  cross  (Fig.  30),  where  the  lacticolor 
female  is  mated  to  the  grossulariata  male,  the  off- 


GR055ULARIATA  9 


LACTICOLOR  if 


FIG.  31. — Abraxas  grossulariata  female  by  A.  lacticolor  male.     The  re- 
ciprocal cross  of  the  one  shown  in  Fig.  30. 

spring  are  all  of  the  grossulariata  type.     When  these 
are  inbred  they  give  (F2)  three  grossulariata  to  one 


86  SEX   INHERITANCE 

lacticolor,  but  the  lacticolors  are  females  only.  The 
lacticolor  grandmother  has  transmitted  her  peculi- 
arity visibly  to  half  of  her  granddaughters,  but  to 
none  of  her  grandsons. 

In  the  reciprocal  cross  (Fig.  31)  of  lacticolor  male 
by  grossulariata  female,  the  daughters  are  like  their 
father  (lacticolor),  and  the  sons  are  like  their  mother 
(grossulariata).  This  is  so-called  criss-cross  inher- 
itance. When  the  hybrids  (Fi)  are  inbred,  they  give 
lacticolor  males  and  females  and  grossulariata  males 
and  females  in  equal  numbers. 

Sex  linked  inheritance,  as  shown  by  the  foregoing 
results,  becomes  intelligible  if  the  factor  for  lacticolor 
is  carried  by  the  chromosome  Z.  Its  occurrence  in 
Z  is  indicated  here  by  writing  an  L  inside  the  circle 
which  represents  that  chromosome,  while  the  allelo- 
morphic  character  carried  by  the  Z  of  the  grossu- 
lariata individual  is  indicated  by  writing  G  in  the 
circle.  The  W  chromosome  is  indicated  by  the 
blank  circle.  The  two  cases  then  work  out  as  shown 
in  the  diagrams. 

The  preceding  analysis  shows  that  the  genetic 
evidence  calls  for  a  mechanism  in  which  the  female  is 
heterozygous  for  sex,  since  those  of  her  eggs  which 
carry  the  factor  for  grossulariata  all  develop  into 
females,  the  others  into  males.  In  the  case  of 
Abraxas  there  was  for  some  years  no  positive  cyto- 
logical  evidence  in  support  of  this  view.  Fortunately, 
the  cytological  side  is  now  in  a  much  better  position 
owing  to  the  work  of  Doncaster  and  Seiler. 

Doncaster  examined  Abraxas   cytologically,   and 


SEX    INHERITANCE  87 

found  that  both  the  female  and  the  male  have  56 
chromosomes,  with  no  obviously  unequal  pair. 

Normally  in  Abraxas  the  sex  ratio  is  about  1  to 
1.  In  one  exceptional  line  this  equality  of  sexes 
was  not  the  rule.  In  this  strain  Doncaster  found 
many  females  which  gave  only  daughters,  and  not  a 
single  son.  Other  females  of  this  line  gave  many 
daughters  but  also  a  few  sons,  while  still  others  gave 
practically  a  normal  1  to  1  ratio. 

When  Doncaster  examined  this  line  cytologically, 
he  found  that  although  the  males  were  normal,  with 
56  chromosomes,  the  females  were  aberrant,  having 
only  55  chromosomes. 

In  the  maturation  of  the  eggs  of  such  a  55  chromo- 
some female,  the  odd  chromosome  went  to  one  pole, 
so  that  one  polar  plate  had  27  and  the  other  28 
chromosomes.  Doncaster  found  further  that  the  odd 
chromosome  went  more  often  to  the  polar  body  than 
to  the  egg.  The  many  eggs  that  eliminate  the  odd 
chromosome  become  after  fertilization  individuals 
with  55  chromosomes,  that  is,  females — while  the 
few  that  retain  it  become  56  chromosome  individuals 
—that  is,  males.  The  preponderance  of  the  females 
is  thus  accounted  for.  Such  females  having  55  chro- 
mosomes would  belong  to  the  OZ  type. 

In  normal  strains  there  is  a  W  chromosome  present, 
but  since  this  W  chromosome  may  be  absent  without 
effect  upon  the  sex  of  the  individual,  as  shown  above, 
it  must  be  regarded  as  functionless  in  determining 
sex,  and  in  this  sense  it  corresponds  to  the  Y  of 
Drosophila.  This  evidence  proves  that  there  is 


88  SEX    INHERITANCE 

present  in  Abraxas  that  cytological  basis  which  the 
evidence  from  sex  linkage  demands,  namely,  a  con- 
dition the  converse  of  that  known  in  other  groups  of 
insects. 

The  evidence  that  Seiler  has  obtained  relates  to  the 
wild  strains  of  the  moth  Phragmatobia  fuliginosa. 
The  reduced  number  of  chromosomes  in  the  polar 
plate  of  the  egg  is  28  (Fig.  32,  a).  The  large  dyad 
formed  by  synapsis  of  the  sex  chromosomes  Z  and  W 
is  shown  in  the  middle  of  the  group.  At  the  first 
polar  division  all  the  chromosomes  separate  from 
their  mates,  the  ordinary  chromosomes  (autosomes) 
as  well  as  the  sex  chromosomes.  But  as  W  separates 
from  Z,  it  breaks  into  two  parts  which  we  may  call 
large  W  and  small  w  (Fig.  32,  6,  c) .  As  a  result  there 
are  29  chromosomes  at  one  pole  (the  pole  that  con- 
tains W  and  w)  and  28  chromosomes  at  the  other 
pole  (the  pole  containing  Z) .  It  is  a  matter  of  chance 
which  group  goes  into  the  polar  body  and  wrhich 
remains  in  the  egg.  Consequently  there  are  two 
kinds  of  eggs,  Ww  and  Z. 

In  the  male  there  are  56  chromosomes,  which  give 
the  reduced  number  28.  The  two  large  Z's  can  be 
made  out  in  Fig.  32 ,  d.  These  meet,  when  the  reduced 
number  28  is  formed,  and  then  separate,  one  going 
to  each  pole  (Fig.  32,  h}.  Each  spermatozoon  con- 
tains, therefore,  one  Z  chromosome. 

FIG.  32. — Phragmatobia  fuliginosa.  a,  equatorial  plate  of  first  polar 
body  of  egg ;  b  and  c,  daughter  plates  of  the  first  polar  spindle ;  d,  equatorial 
plate  of  spermatogonium ;  e,  equatorial  plate  of  first  spermatocyte;  / 
and  g,  equatorial  plates  of  second  spermatocyte;  A,  anaphase  stage  of 
first  maturation ;  t  and  j,  equatorial  plates  of  somatic  cells  with  56  (in  i), 
and  61  chromosomes  (in  j).  (After  Seiler.) 


SEX    INHERITANCE 


89 


•  »*••••* 

*  •••• 


*• 


8* 


FIG.  32. 


90  SEX    INHERITANCE 

Any  sperm  fertilizing  an  egg  containing  Ww  pro- 
duces a  female.  The  male  embryos  should  contain 
therefore  56  chromosomes,  the  female  57.  Counts 
of  chromosomes  in  embryos  show  that  while  some 
contain  56,  others  contain  58,  61  and  62.  Seiler 
suggests  that  the  Z  element  is  also  compound  and 
sometimes  separates  into  four  components  in  the 
somatic  cells.  Aside  from  this  peculiarity  his  results 
indicate  that  the  male  is  monogametic  and  the  female 
digametic  in  accordance  with  the  WZ-ZZ  scheme. 

In  other  Lepidoptera,  examined  by  Stevens,  by 
Doncaster,  by  Dederer  and  by  Seiler,  the  males  and 
females  have  the  same  chromosome  configuration. 
In  other  words,  if  a  WZ  pair  is  present  "in  the  female 
the  members  are  of  the  same  size,  or  so  nearly  of  the 
same  size  that  they  cannot  be  distinguished.  It 
will  be  recalled  that  in  a  few  other  insects,  believed 
for  other  reasons  to  belong  to  the  Drosophila  type, 
the  X  and  the  Y  chromosomes  are  of  the  same  size. 
The  failure  to  find  two  sizes  of  sex  chromosomes  in 
moths  is,  therefore,  not  an  argument  against  the  view 
that  the  female  is  heterozygous  for  a  sex  factor.  On 
the  contrary,  it  is  to  be  considered  only  a  fortunate 
circumstance  that  this  difference  in  a  sex  factor  is 
sometimes  associated  with  a  size  difference  in  no  way 
directly  depending  on  the  sex  factor  itself. 

WHAT  ARE  SEX  FACTORS 

The  inheritance  of  sex  is  explained  by  the  assump- 
tion that  one  difference  distinguishes  the  male  from 


SEX    INHERITANCE  91 

the  female;  the  difference  is  a  single  or  a  double 
amount  of  the  so-called  sex  factor.  The  chromo- 
somes are  the  carriers  of  these  sex  factors.  The 
symbols  used  here,  viz.,  XX-XY  and  WZ-ZZ,  are 
intended  primarily  for  the  chromosomes,  but  also 
for  the  sex  factors. 

These  formulae  for  the  Drosophila  type  and  for 
the  Abraxas  type  raise  the  question  as  to  whether 
the  postulated  sex  factors  are  identical  in  the  two 
cases.  The  employment  of  different  letters  for  the 
two  types  suggests,  of  course,  that  the  sex  factors 
may  be  different.  And  it  is  true  that  the  two  sets 
of  letters  are  used  to  avoid  an  apparent  paradox 
that  appears  if  we  use  only  X  and  Y  in  both  cases. 
If  this  is  done,  XY  on  one  scheme  represents  the 
male  and  on  the  other  scheme  the  female.  Never- 
theless, for  the  present  the  employment  of  different 
letters  need  not  necessarily  mean  that  different 
factors  for  sex  are  present  in  the  two  great  classes, 
for  these  reverse  results  may  be  due  to  the  action  of 
the  same  factor-difference  in  a  different  setting. 
For  example,  acid  may  be  the  color  differentiator  in 
a  setting  of  a  certain  solution  containing  it  and  litmus 
(with  one  drop  of  acid  the  color  being  blue,  with 
two  drops  red) ,  but  in  a  setting  containing  Congo 
red  the  same  differentiator  may  produce  just  the 
opposite  effects  (one  drop  red,  two  drops  blue).  On 
the  other  hand,  it  is  conceivable  that  the  setting  (lit- 
mus and  acid)  may  remain  the  same  and  yet  a  reverse 
result  be  produced  by  having  a  different  differentiator 
— alkali  instead  of  acid. 


92  SEX    INHERITANCE 

Since  genetics  has  at  present  nothing  to  offer  that, 
will  decide  the  question  as  to  whether  another  set  of 
sex  differentiators  is  present,  or  whether  the  same 
differentiators  with  a  different  setting  are  involved 
in  these  two  cases,  discussion  is  quite  certain  to  be 
futile. 

It  may  seem  inconsistent  to  use  the  name  of  the 
chromosome  as  the  symbol  for  the  sex  factor  when 
dealing  with  the  inheritance  of  sex,  while  in  all  other 
cases  a  factor  representing  a  point  in  the  chromosome 
is  used  to  designate  the  special  character  under  con- 
sideration. No  doubt  with  this  idea  in  mind,  several 
writers  have  followed  the  practice  of  indicating  the 
sex  factor  by  a  significant  letter,  such  as  F  for  female- 
ness  and  M  for  maleness.  As  the  use  of  such  letters 
often  involves  a  question  of  interpretation,  a  brief 
consideration  may  be  given  to  this  matter.  In  the 
discussion  that  follows  reference  is  made  always  to 
the  Drosophila  type,  but  exactly  the  same  arguments 
apply  to  the  Abraxas  type. 

1.  It  has  been  suggested,  for  example,  that  a  factor 
for  the  male  be  added  to  the  formulae  so  that  maleness 
may  not  appear  simply  as  the  absence  of  one  factor 
for  femaleness.  Thus,  in  such  formula  as  FMFM 
( $ )  and  FMM  ( $ )  the  factor  for  maleness  is  added 
to  indicate  that  when  a  single  amount  of  F  is  present 
the  male  factors  produce  the  male.  But  since  M's 
are  distributed  everywhere,  the  formula  is  little  more 
than  a  concession  to  male  vanity,  for  M  is  not  here 
a  differentiator.  Moreover,  the  use  of  the  letters 
MM  is  here  unjustifiable  because  there  is  no  ground 


SEX    INHERITANCE  93 

for  supposing  that  maleness  is  due  to  one  pair  of 
factors.  It  must  be  due  to  a  complex  of  many  factors 
all  of  which  are  present  in  both  sexes. 

2.  Since  there  is  evidence  to  show  in  some  cases 
that  there  is  no  sex  factor  in  the  Y  chromosome,  the 
factor  or  factors  carried  by  X  can  have  no  mate  in 
this  sex,  hence  the  allelomorph  or  allelomorphs  must 
be  0.     If   one   chooses  to  represent  this  zero  by  a 
small  letter,  by  f  or  m  for  instance,  there  is  no  in- 
consistency in  doing  so,  for  there  is  in  this  instance 
the  cytological  observation  to  justify  its  use.     It  is, 
however,  misleading  to  represent  this  0  by  M  as  has 
sometimes  been  done. 

3.  There  is  at  present  no  evidence  to  show  that 
there  is  only  one  factor  for  sex  carried  by  each  sex 
chromosome,  however  probable  this  may  seem  from 
other  relations,  for  it  has  not  been  possible  to  de- 
termine the  linkage  relation  of  the  sex  factor  or  fac- 
tors to  other  factors  in  the  sex  chromosome,  because 
crossing  over  of  like  factors  in  the  homozygous  sex 
would  lead  to  no  visible  effect,  and  in  the  heterozy- 
gous sex  no  crossing  over  takes  place. 

4.  If  in  the  formulae  FF  ( ?)  and  FO  (£)  the  letter 
F  is  interpreted  as  a  factor  for  femaleness,  the  formula 
must  not  be  construed  as  meaning  that  F  may  not 
also  be  a  factor  for  maleness.     For,  as  a  matter  of 
fact,  one  F  factor  may  be  essential  to  the  production 
of  the  male.     Therefore,  until  we  get  more  definite 
information  as  to  the  existence  of  a   single   or  of 
several  factors  for  sex,  and  as  to  whether  they  are 
the  same  factors  in  the  two  types,  and  what  the  rela- 


94  SEX    INHERITANCE 

tion  of  F  and  M  may  be  in  hermaphroditic  types,  it  is 
less  inconsistent  to  use  the  symbols  for  the  sex  chro- 
mosomes as  the  symbols  for  the  sex  factors  also,  if 
it  is  at  the  same  time  recognized  that  the  whole 
chromosome  is  not  involved  in  determining  sex. 

The  same  factors  that  determine  whether  eggs  or 
sperm  develop  in  an  individual  must  also  be  respon- 
sible for  the  development  of  many  characters  that  go 
along  with  the  male  or  the  female  condition,  in  other 
words  those  characters  that  are  different  in  the  two 
sexes  (sex  limited).  Sex  factors  of  whatever  kind, 
however,  must  like  all  factors  be  supposed  to  produce 
their  effects  in  conjunction  with  the  rest  of  the  cell, 
with  other  factors,  or  writh  anything  else  there,  for  it 
must  always  be  remembered  that  the  sex  factor  is 
only  one  of  many  factors  that  are  at  work.  Hence 
for  the  realization  of  any  particular  character  that  is 
associated  with  a  particular  sex  there  are  probably 
many  factors  that  cooperate.  If  the  latter  change, 
the  character  in  question  may  also  change,  while  the 
sex  factor  remains  the  same.  The  character  may, 
in  this  case,  be  said  to  be  dependent  not  only  on  the 
sex  factor,  but  also  .on  another  differentiator  that 
can  only  realize  itself  conjointly  with  the  sex  factor. 
Thus  while  the  accessory  sexual  organs,  as  well  as  the 
secondary  sexual  characters  and  all  other  sex-limited 
characters,  may  be  modified  by  special  differentiators 
that  are  not  present  in  the  sex  chromosomes,  yet 
the  sex  factor  also  produces  an  effect  on  their  de- 
velopment which  is  different  according  to  whether 
the  sex  factor  exists  in  single  or  double  amount.  In 


SEX    INHERITANCE  95 

the  case  of  some  other  characters,  however,  it  is 
conceivable  that  the  sex  factors  co-operate  in  their 
production,  and  yet  have  the  same  effect  whether 
present  in  single  or  double  amount.  Such  characters 
would  not  be  sex  limited. 

As  in  the  case  of  sex  limited  characters,  so  in  the 
case  of  sex  itself  there  must  be  many  factors  in  the 
fertilized  egg  that  are  as  essential  to  the  development 
of  sex  as  are  the  sex  factors  themselves,  but  as  they 
are  distributed  to  all  individuals  alike,  they  are  not 
thought  of  as  differentiators,  but  as  forming  the 
chemical  background  on  which  the  single  or  the 
double  amount  of  the  sex  factor  gives  its  result.  It 
is  quite  conceivable  that  one  or  more  of  these  other 
factors  might  so  change  that  the  sex  differentiators 
would  become  inoperative  or  even  change  so  that 
these  other  factors  themselves  become  the  differen- 
tiators that  determine  sex. 

The  environment — the  outer  world — is  also  one  of 
the  components  that  enters  into  the  development  of 
every  individual.  A  specific  environment  is  one  of 
the  conditions  of  development.  Why  then,  it  may 
be  asked,  may  not  the  environment  turn  the  scale 
and  determine  sex?  As  a  general  proposition  this 
must  be  acceded  to  at  once — it  is  entirely  a  matter  of 
proof.  If  there  is  an  internal  mechanism  to  de- 
termine sex  in  a  normal  environment  it  is  quite  con- 
ceivable that  it  might  be  supplanted  in  a  new  world: 
It  is  a  question  of  evidence  as  to  how  often,  if  ever, 
this  occurs.  It  is  furthermore  quite  conceivable  that 
some  animals  have  no  internal  mechanism  to  regulate 


96  SEX    INHERITANCE 

sex  but  depend  on  a  difference  in  their  medium.  If 
such  an  environment  can  be  discovered  it  would  be 
sex  determining  in  the  same  sense  in  which  the  term 
is  here  employed  when  the  sex  differentiators  are 
hereditary  factors. 

Sex  determination  in  the  Gephyrean  worm  Bonellia 
is  a  case  in  point.  The  female  is  a  large  oval  worm 
with  a  long  proboscis.  The  male  is  very  small  and 
degenerate  and  lives  as  a  parasite  on  the  proboscis 
of  the  female.  The  development  has  recently  been 
studied  by  Baltzer.  He  finds  that  if  the  young 
Bonellia  embryos  are  put  into  an  aquarium  in  which 
old  females  are  present,  they  settle  down  on  the 
proboscis  of  the  female  and  degenerate  into  males. 
If,  on  the  contrary,  the  young  embryos  are  kept  by 
themselves  they  pass  through  an  indifferent  stage 
but  later  differentiate  into  female  worms.  Whether 
a  male  or  a  female  develops  from  an  egg  depends 
here  on  whether  at  a  certain  stage  the  embryo  comes 
under  the  influence  of  the  proboscis  of  a  female  or 
fails  to  do  so.  Some  secretion  from  the  proboscis 
may  be  the  differentiator  in  such  a  case.  It  is  clear 
that  here  it  is  environment  that  determines  the 
sex  of  the  individual.  The  evidence  suggests  that 
the  male  organs  develop  first  in  the  presence  of  a 
certain  secretion  from  the  proboscis  of  the  female 
which  also  serves  to  arrest  the  animals  in  this  stage. 
If,  however,  the  animal  fails  to  meet  with  these 
conditions,  it  usually  ceases  for  a  time  to  develop 
and  fails  to  produce  the  male  organs.  Later  it 
starts  once  more  to  go  forward  and  develops  the 


SEX    INHERITANCE  97 

female  organs  which  are  characteristic  of  this  sex. 
Baltzer  found,  however,  that  only  90  per  cent,  of 
these  free  embryos  became  females;  the  remaining 
10  per  cent,  developed  into  hermaphrodites.  He 
speaks  of  Bonellia  as  a  protandric  hermaphrodite  in 
which  one  or  the  other  sort  of  reproductive  organs 
may  be  suppressed  by  the  environment,  but  this  is 
only  another  way  of  describing  the  results. 

There  are  several  groups  in  which  a  change  from 
parthenogenesis  to  sexual  reproduction  takes  place 
in  response  to  changes  in  the  environment.  The 
best  known  cases  are  the  rotifer  (Hydatina  senta), 
some  of  the  daphnians  (Moina  and  Simocephalus) 
and  certain  insects  (aphids).  Hydatina  gives  the 
clearest  evidence  (Fig.  33).  It  has  been  shown  by 
Whitney  and  by  A.  F.  Shull  that  if  this  rotifer  is  fed 
on  a  colorless  flagellate  and  kept  in  water  from  old 
cultures  it  can  be  kept  indefinitely  reproducing  by 
parthenogenesis,  i.e.,  by  eggs  that  are  not  fertilized. 
If  taken  out  of  these  solutions  and  put  into  spring 
water,  a  certain  percentage  of  the  females  will  give 
rise  to  daughters  whose  eggs  may  be  fertilized.  These 
daughters  behave  therefore  as  sexual  females.  But 
if  they  are  not  impregnated  their  eggs  remain  viable 
and  develop  parthenogenetically  into  males.  If  in 
addition  to  being  transferred  to  spring  water  the 
females  are  fed  with  a  green  flagellate,  Dunaliella, 
then,  as  Whitney  has  shown,  almost  all  of  their 
daughters  (80  per  cent.)  are  changed  into  the  sexual 
form,  i.e.,  a  form  producing  eggs  capable  of  being 
fertilized  (or  if  not  fertilized,  developing  into  males) . 


98 


SEX    INHERITANCE 


An  environmental  change  determines  whether  par- 
thenogenesis or  sexual  reproduction  takes  place. 
The  environment  may  equally  well  be  said  to  de- 


FIG.  33. — Diagram  to  illustrate  the  life  cycle  of  Hydatina  senta.  The 
environment  determines  whether  the  parthenogenetic  individual  (at  the 
top)  gives  rise  to  a  female  like  herself ,  or  to  one  that  if  fertilized  at  an 
early  age  produces  a  sexual  egg,  but  if  not  fertilized  produces  small  eggs 
from  which  males  develop  parthenogenetically. 

termine  whether  an  egg  becomes  a  parthenogenetic 
female-producing  female  or  a  male-producing  female. 


SEX    INHERITANCE 


99 


But  another  difference  is  necessary  to  determine 
whether  an  egg  of  the  latter  individual  develops  into 
a  male  or  into  a  female,  namely,  the  entrance  of  a 
spermatozoon  into  the  egg  before  it  has  completed 
its  growth. 


FIG.  34. — Diagram  to  illustrate  the  life  cycle  of  Phylloxera  carysecaulis. 

In  the  phylloxerans  of  the  hickories  the  fertilized 
egg  gives  rise  to  a  female  called  the  stem  mother 
(Fig.  34).  She  emerges  from  the  egg  in  the  early 
spring  and  attaches  herself  by  means  of  her  proboscis 
to  a  leaf,  causing  it  to  produce  a  gall  that  envelops 
her.  Within  the  gall  she  lays  her  eggs.  These 
hatch,  and  produce  the  winged  or  migrant  generation 


100  SEX    INHERITANCE 

(Fig.  34).  In  one  species,  P.  caryaecaulis,  all  the  mi- 
grants in  one  gall  are  alike  in  that  they  produce  the 
same  kind  of  egg,  i.e.,  in  some  galls  all  the  migrants 
contain  large  eggs  (that  produce  sexual  females), 
while  in  other  galls  all  of  the  migrants  contain  smaller 
eggs  (that  develop  into  males). 

The  sexual  female  lays  one  egg,  that  is  fertilized, 
from  which  the  stem  mother  emerges  the  following 
spring.  The  males  give  rise  only  to  female-producing 
sperm,  each  spermatozoon  containing  two  sex  chromo- 
somes. The  other  class  of  sperm  degenerates.  Hence 
we  can  understand  why  it  is  that  all  fertilized  eggs 
produce  females  only. 

The  chromosomal  cycle  undergoes  the  series  of 
changes  shown  in  Fig.  35.  In  P.  carysecaulis  there 
are  eight  chromosomes,  including  four  sex  chromo- 
somes (XxXx) .  Since  the  history  of  the  sex  chromo- 
somes alone  furnishes  certain  information  that  makes 
clear  some  of  the  changes  in  the  life  cycle,  the  other 
chromosomes  may  be  disregarded  for  the  present. 

Starting  at  the  bottom  of  the  diagram  it  will  be 
seen  that  the  sexual  egg  after  extruding  the  two  polar 
bodies  contains  two  sex  chromosomes  indicated  by 
X  and  x.  Two  kinds  of  males  are  indicated  in  the 
diagram,  one  containing  Xx  the  other  Xx',  and  as  a 
consequence  there  will  be  two  kinds  of  female-produc- 
ing sperm,  one  kind  for  each  male,  namely,  Xx  and 
Xx'.  If  the  former  fertilizes  the  sexual  egg,  the  re- 
sulting stem  mother  will  be  XxXx,  if  the  latter, 
the  stem  mother  will  be  XxXx'.  These  two  kinds  of 
stem  mothers  are  indicated  at  the  top  of  the  diagram. 


SEX    INHERITANCE 


101 


One  of  them,  XxXx,  produces  eggs  which,  after 
extruding  one  polar  body,  give  rise  to  the  migrants 
bearing  large  eggs;  from  the  latter  eggs,  in  turn,  come 


XxX: 


3TtM  nOTHER-FtmLCTROOUCING  UNE 


XxXx' 

arm  none*  -  MALC-ntoouoNC  UNE 


XxX: 


MIGRANT -FEMALE  PRODUCER 


XxX 


2""  GENERATION 


XxXx 

FEMALE 


Xx 

HALE-TYPE  I 


Xx' 

MALE. TYPE  H 


POLAR    SPINDLE 


FEttAI-E  PRODUCING 
5PCRM 


FIG.  35. — Diagram  to  illustrate  the  chromosomal  cycle  of  Phylloxera 

caryaecaulis. 


102  SEX    INHERITANCE 

the  sexual  females.  The  other  stem  mother  XxXx', 
produces  eggs,  which,  after  extruding  one  polar 
body,  give  rise  to  the  migrants  bearing  small  eggs. 
Prior  to  the  time  when  these  small  eggs  are  about  to 
give  off  their  single  polar  body,  the  two  large  X's 
conjugate  and  the  two  small  x's  conjugate,  and  when 
the  polar  body  is  given  off  one  large  and  one  small  X 
pass  out,  and  one  large  and  one  small  X  remain  in  the 
egg.  In  other  words  there  is  at  this  time  a  reduction 
in  the  number  of  sex  chromosomes,  and,  as  a  conse- 
quence, a  male  is  produced.  Now  as  the  diagram 
shows,  Xx  may  remain  in  the  egg  and  Xx'  pass  out ; 
or,  in  other  eggs,  Xx'  may  remain  in  the  egg  and  Xx 
may  pass  out.  There  will  be,  in  consequence,  two 
kinds  of  males,  one  Xx,  the  other  Xx',  and  as 
stated,  two  kinds  of  female  producing  sperm  Xx 
and  Xx'. 

Thus  the  life  cycle  is  brought  back  to  the  starting 
point.  It  may  be  added  that  so  far  as  the  chromo- 
somes other  than  the  X  chromosomes  are  concerned 
there  is  no  synapsis  and  no  reduction  to  the  haploid 
number  in  either  line  until  the  maturation  divisions 
of  the  third  or  sexual  generation  occur.  The  life 
cycle  of  this  species  illustrates  three  points: 

First. — That  all  of  the  sperm  are  female  producing, 
because  the  male-producing  class  of  sperm  degener- 
ates, as  has  been  shown  by  direct  observation. 

Second. — That  the  parthenogenetic  females  can 
produce  males  through  the  elimination  of  two 
chromosomes.  The  female  contains  four  sex  chro- 
mosomes and  the  male  two.  The  elimination  of  the 


SEX    INHERITANCE  103 

two  chromosomes  in  the  polar  body  of  the  male- 
producing  egg  has  been  directly  demonstrated. 

Third. — In  this  species  the  somewhat  unusual 
relation  of  one  stem  mother  giving  rise  to  the  line 
that  culminates  in  the  sexual  eggs,  and  of  another 
stem  mother  giving  rise  to  the  line  that  culminates 
in  the  males,  can  be  explained  on  the  assumption 
that  one  pair  of  the  sex  chromosomes  is  heterozygous 
in  some  factor  indicated  in  the  diagram  by  priming 
one  of  the  x's.  This  explanation  is  in  part  theoret- 
ical, although  it  is  based  on  the  actual  observation 
of  two  kinds  of  males  that  differ  in  respect  to  the 
behavior  of  one  of  the  smaller  x's. 

In  other  species  of  phylloxerans,  and  in  many 
aphids,  one  stem  mother  may  produce  both  lines, 
i.e.,  some  of  her  offspring  may  ultimately  give  rise 
to  sexual  females  and  others  to  males.  In  such 
cases,  as  is  illustrated  in  the  next  diagram  (Fig.  36), 
there  is  but  one  kind  of  stem  mother,  and  the  four 
sex  chromosomes  (there  are  only  two  sex  chromo- 
somes in  the  aphids)  are  alike.  Here  some  environ- 
mental influence  must  determine  that  in  certain  eggs 
conjugation  of  two  pairs  of  chromosomes  takes  place. 
Such  eggs  give  rise  to  males.  In  other  eggs  where 
this  does  not  take  place  the  sexual  female  will  be 
produced. 

In  both  P.  carya3caulis  and  in  P.  fallax,  and  also  in 
the  other  forms  referred  to  above,  the  difference 
between  the  behavior  of  the  chromosomes  in  the 
stem  mother  and  in  the  male-producing  migrant  is 
dependent  on  environmental  influences.  On  the 


104 


SEX    INHERITANCE 


other  hand,  the  difference  between  the  eggs  contained 
in  the  male-producing  and  in  the  female-producing 
migrants,  and  between  the  behavior  of  the  chro- 

xxxx 


--r«      *-~-_rc 


XXXX 

MCRAVT- FEMALE  PRODUCER 


XXXX 

MOUNT.- MALE  PRODUCER 

XX     XX 


XX     XX 

FEMALE 


XX 


XX 


FIG.  36. — Diagram  to  illustrate  the  chromosomal  cvcle  of  Phylloxera 

fallax. 


SEX    INHERITANCE  105 

mosomes  in  these  eggs,  depends  in  P.  caryaecaulis  on 
an  initial  chromosomal  difference  between  the  types 
of  migrants,  but  in  P.  fallax  on  some  environmental 
influence.  The  difference  between  the  two  kinds  of 
sexual  individuals,  i.e.,  sex  itself,  is  determined  in 
both  cases  by  the  distribution  of  the  chromosomes, 
however  that  distribution  may  itself  be  conditioned. 

It  is  scarcely  necessary  to  speak  of  other  cases,  in 
which,  although  an  internal  mechanism  is  known  to 
exist  for  producing  equal  numbers  of  males  and 
females,  yet  more  individuals  of  one  or  the  other 
sex  may  actually  appear.  For  instance,  it  has  been 
shown  repeatedly  in  Drosophila  that  when  a  sex 
linked  lethal  factor  is  present  in  the  sex  chromosome, 
any  male  that  contains  this  lethal  X  chromosome 
will  perish.  The  females,  on  the  other  hand,  will 
live,  because  they  contain  in  addition  another  X 
chromosome,  having  the  dominant  normal  allelo- 
morph of  the  lethal  factor.  These  lethal  factors 
may  be  factors  that  cause  abnormalities  in  organs 
essential  to  the  life  of  the  individuals  and  destroy  the 
individual  in  this  sense.  The  changed  ratios  do  not 
at  all  affect  the  theory  that  there  exists  in  Drosophila 
an  internal  sex-determining  mechanism,  although 
were  the  cases  not  actually  worked  out,  the  abnormal 
ratios  might  have  seemed  to  disprove  the  theory  of 
the  sex  chromosomes. 

We  can  imagine  other  ways  in  which  even  in  the 
presence  of  a  regulating  sex  mechanism  the  actual 
ratio  of  males  to  females  might  be  changed  from 
equality  to  some  other  ratio.  For  instance,  since  the 


106  SEX    INHERITANCE 

female-producing  spermatozoa  contain  one  more 
chromosome  and  are  larger  in  consequence,  as  Zeleny 
and  Faust  have  shown,  such  sperm  might  travel  up 
the  oviduct  with  less  speed  than  the  male-producing 
sperm.  Hence  the  sex  ratios  would  be  affected  in 
favor  of  the  males.  Furthermore,  secretions  in  the 
oviduct  might  act  differently  on  the  two  kinds  of 
sperm,  the  age  of  the  sperm  might  affect  one  kind 
more  than  the  other,  etc.  Such  effects  would  be 
expected  to  bring  about  deviations  from  the  normal 
ratios,  but  effects  of  these  kinds  can  not  fairly  be 
brought  forward  to  disprove  the  hypothesis  that  the 
X-bearing  spermatozoa  give  rise  to  females  and  the 
no-X-bearing  (or  Y)  spermatozoa  give  rise  to  males. 
It  has  even  been  suggested  that  external  conditions 
might  so  weaken  or  strengthen  the  X  chromosome 
that  an  X-bearing  spermatozoon  might  produce  a 
male  or  that  a  no  X-bearing  sperm  might  produce  a 
female.  If  such  effects  can  be  produced  they  would 
act,  no  doubt,  in  the  way  postulated.  But  there  is  a 
large  amount  of  evidence  showing  that  factors  are 
not  ordinarily  altered  by  environmental  influences. 
Nevertheless  there  is  no  conflict  here  with  the  sex 
chromosome  mechanism,  only  another  one  is  imagined 
to  have  the  power  to  overthrow  it.  No  adherent  of 
the  chromosome  theory  would  deny  the  theoretical 
possibility  that  at  times  external  conditions  may 
at  least  overcome  the  usual  effect  of  the  sex  factor 
if  not  the  sex  factor  itself,  but  the  burden  of  proof  for 
such  supposed  reversal  of  the  normal  result  lies  with 
those  who  maintain  it.  Proof,  if  it  were  forthcom- 


SEX    INHERITANCE  107 

ing,  that  the  machinery  of  sex  determination  may  be 
upset  is  not  an  argument  against  the  sex  chromosome 
theory.  It  is  not  a  refutation  of  the  factorial  hy- 
pothesis of  sex  determination,  for  in  such  a  case  there 
would  only  be  a  substitution  of  an  environmental 
factor  for  a  genetic  one.  Opponents  and  advocates 
of  the  chromosome  theory  of  sex  determination  have 
often  failed  to  realize  that  "f actor  for  sex"  is  not 
used  in  an  absolute  sense,  but  as  the  best  known  or 
most  usual  factor-difference  among  any  number  of 
possible  theoretical  ones;  and  in  consequence  the 
identification  of  sex  as  a  character  with  the  factor 
for  sex  determination  has  led  to  needless  confusion. 
If  the  factor  for  sex  were  identified  with  sex  itself, 
i.e.,  if  it  alone  would  produce  sex,  there  would  be  of 
course  only  one  form  of  sex  determination  possible. 
But,  as  no  one  maintains  such  an  interpretation  of  sex 
determination,  this  view  can  not  be  properly  advanced 
as  an  argument  against  the  sex  chromosome  theory. 


CHAPTER  V 

THE  CHROMOSOMES  AS  BEARERS  OF 
HEREDITARY  MATERIAL 

The  evidence  in  favor  of  the  view  that  the  chromo- 
somes are  the  bearers  of  hereditary  factors  comes  from 
several  sources  and  has  continually  grown  stronger, 
while  a  number  of  alleged  facts,  that  seemed  opposed 
to  this  evidence,  have  either  been  disproven,  or  else 
their  value  has  been  seriously  questioned.  We  pro- 
pose now  to  examine  in  some  detail  the  observations 
and  experiments  that  bear  on  the  chromosome  theory 
of  heredity. 

THE  EVIDENCE  FROM  EMBRYOLOGY 

Relating    to    the   Influence    of  the    Chromosomes   on 
Development 

It  has  been  argued  that  since  the  sperm  transmits 
equally  with  the  egg,  and  since  only  the  sperm  head, 
consisting  of  the  nucleus,  enters  the  egg,  inherit- 
ance is  only  through  the  nucleus.  But  it  must  be 
admitted  that  around  the  entering  sperm  nucleus 
there  may  be  a  thin  enveloping  protoplasm,  which, 
however  scanty,  might  suffice  to  transmit  certain 
cytoplasmic  factors.  Moreover,  while  the  tail  of 
the  sperm  appears  in  some  cases  to  be  left  outside  the 

108 


THE    CHROMOSOMES  109 

egg,  in  other  cases  it  appears  to  enter  and  to  be 
absorbed. 

Behind  the  head  of  the  spermatozoon,  and  at  the 
base  of  the  tail,  there  is  a  middle  piece  which  contains 
a  derivative  of  the  old  centriole  or  division  center. 
Since  the  centrosome  carried  by  the  sperm  has  been 
found  in  some  forms  to  give  rise  to  the  new  centro- 
somes  that  occupy  the  poles  of  the  first  cleavage  spindle 
of  the  egg,  it  may  appear  that  a  paternal  contribu- 
tion can  come  about  in  this  way.  It  is  true  that  the 
continuity  of  the  centrosome  of  the  sperm  with  that 
of  the  dividing  egg  has  been  disputed  in  some  forms; 
but  it  is  difficult  to  prove  that  the  sperm  centrosome 
is  lost,  even  though  it  may  disappear  owing  to  loss 
of  staining  power. 

The  nucleus  contains  a  sap  which  is  probably  of 
cytoplasmic  origin.  The  presence  of  this  sap  may 
again  be  appealed  to  by  those  who  do  not  accept 
the  chromosomes  as  the  bearers  of  heredity,  as  a 
weak  link  in  the  evidence.  It  is  true  that  the  nuclear 
sap  appears  to  be  squeezed  out  of  the  nucleus  of  the 
sperm  head,  leaving  a  compact  and  apparently  solid 
mass  of  chromatin,  yet  its  complete  elimination  can 
not  be  proved.  Hence,  while  those  who  favor 
chromosomal  transmission  find  in  the  facts  of  normal 
fertilization  strong  indications  favorable  to  that 
view,  yet  it  is  also  true  that  those  who  are  inclined  to 
dispute  this  view  find  several  loopholes  in  the 
argument  of  their  opponents. 

The  importance  of  the  nucleus  in  heredity  has 
further  been  shown  by  experiments  of  Bierens  de 


110  THE    CHROMOSOMES 

Haans,  Herbst,  and  Boveri  on  giant  eggs  of  sea 
urchins  fertilized  by  sperm  of  another  species.  The 
hybrid  larvae  produced  when  normal  eggs  of  one 
species  are  fertilized  by  sperm  of  the  other  species 
are  intermediate  in  character  between  the  two 
parental  types. of  larvae;  while  those  from  giant  eggs 
of  the  same  species  fertilized  by  sperm  of  the  other, 
also  intermediate,  incline  more  to  the  maternal  side. 
The  nucleus  of  the  giant  egg  is  double  the  size  of 
that  of  the  normal  egg  and  according  to  Bierens  de 
Haans  the  chromosomes  are  also  double  in  number. 
Consequently,  the  amount  of  maternal  chromatin 
should  be  double  that  introduced  by  the  sperm,  and 
might  produce  a  corresponding  influence  on  the 
hybrid  character.  But  since  in  these  giant  eggs  the 
cytoplasm  is  also  doubled,  it  is  not  evident  that  the 
results  are  due  to  the  chromosomes  rather  than  to  the 
cytoplasm.  By  means  of  the  following  ingenious 
comparison  Boveri  has  shown  that  the  results  must 
be  ascribed  to  the  chromosomes  rather  than  to 
cytoplasm.  Normal  eggs  were  broken  into  frag- 
ments, the  nucleated  pieces  were  fertilized  with  the 
sperm  of  the  other  species,  and  those  fragments  of 
half  the  volume  of  the  normal  egg  were  isolated. 
As  is  known,  such  fragments  develop  into  whole 
larvae,  whose  nuclei  will  have  the  usual  chromatin 
content.  The  egg  cytoplasm  is,  however,  reduced  to 
half.  Nevertheless  the  larvae  did  not  incline  to  the 
paternal  side,  although  these  larvae,  like  all  larvae 
from  fragments,  were  often  simpler  than  the  normal. 
Hence  since  a  relative  decrease  in  the  amount  of 


THE    CHROMOSOMES  111 

cytoplasm  does  not  here  affect  the  character  of  the 
larvae,  it  is  rational  to  suppose  that  an  increase  such 
as  is  present  in  the  giant  eggs  likewise  produces  no 
such  effects  as  observed  in  the  larvae.  At  the  same 
time,  normal  eggs  were  cross  fertilized  and  in  the 
two-cell  stage  the  blastomeres  were  separated.  The 
contributions  by  the  two  parents  were  relatively  the 
same  as  in  the  normal  egg.  These  larvae  were  like 
those  from  egg  fragments,  and  serve  as  a  control  of 
those  larvae  in  so  far  as  they  bear  on  the  question  of 
how  far  size  alone  may  affect  the  result.  Moreover, 
in  them,  the  relation  of  the  chromosomes  to  the 
cytoplasm  is  the  same  as  in  the  normal  egg  (whether 
the  sperm  does  or  does  not  bring  in  cytoplasm). 
Hence,  since  the  amount  of  cytoplasm  is  shown  to 
have  no  influence  on  the  character  of  these  larvae, 
there  is  no  reason  for  supposing  that  it  had  any 
influence  in  the  case  of  the  giant  eggs. 

Boveri's  studies  upon  dispermic  fertilization  of 
the  egg  of  the  sea  urchin  bear  directly  upon  the 
question  at  issue.  He  found  that  when  two  sperm 
simultaneously  enter  the  same  egg,  each  brings  in  a 
centrosome,  so  that  a  tetra-  or  tri-polar  spindle  is 
formed  for  the  first  division,  as  shown  in  Fig.  37. 
Instead  of  a  double  set  of  chromosomes,  as  in  normal 
fertilization,  there  are  three  sets.  At  the  first 
division,  the  chromosomes  are  irregularly  distrib- 
uted upon  the  multipolar  spindles.  In  consequence, 
some  cells  may  get  one  of  each  kind  of  chromosome, 
while  other  cells  may  get  less  than  a  full  complement 
(Fig.  38).  These  dispermic  eggs  almost  always  give 


112 


THE    CHROMOSOMES 


rise  to  abnormal  embryos,  as  several  observers  have 
recorded.  The  result  can  best  be  attributed  to  the 
irregular  distribution  of  qualitatively  different  chro- 
mosomes; only  those  embryos  in  which  each  cell  has  a 
full  complement  developing  normally. 

Boveri's  evidence  went  still  further,  for  he  sepa- 
rated the  first  cleavage  cells  of  these  dispermic  eggs 


FIG.  37. — Dispermic  fertilization  of  egg  of  sea  urchin.  The  four 
centrosomes  cause  an  unequal  distribution  of  the  fifty-four  chromosomes, 
leading  at  the  first  division  to  four  cells  which  contain  different  num- 
bers of  chromosomes. 


and  followed  their  history.  Some  of  them  gave  rise 
to  perfect  dwarf  larvae.  The  number  of  normal 
embryos  was  small,  but  was  that  expected  on  the 
chance  distribution  of  the  chromosomes,  for  we 
should  expect  to  find  in  a  few  cases  an  isolated  cell 
that  contained  a  full  complement  of  chromosomes 
and  from  such  a  cell  a  normal  embryo  would  be 
formed.  The  abnormality  in  development  of  the 
rest  of  the  isolated  cells  was  not  due  to  any  harmful 


THE    CHROMOSOMES 


113 


effect  caused  by  isolation,  for  it  had  been  shown  by 
Driesch  and  others  that  when  the  first  two  cells  of  a 
sea-urchin  egg  that  has  been  normally  fertilized  are 
separated,  each  forms  a  perfect  embryo.  Such  cells, 
although  containing  only  half  the  cytoplasm,  contain 


FIG.  38. — Diagram  to  show  five  combinations  of  chromosomes  result- 
ing from  the  first  division  of  dispermic  eggs,  in  which  either  each  cell  gets 
one  complete  set  of  chromosomes,  a;  or  three  cells  get  a  full  set,  6;  or 
two  cells,  c;  or  one  cell,  d;  or  none  of  the  four  cells,  e,  get  a  full  set. 
(After  Boveri.) 

a  full  set  of  chromosomes.  The  difference,  therefore, 
between  these  cells  and  isolated  cells  from  dispermic 
eggs  would  seem  to  be  due  mainly  to  their  different 
chromosomal  contents. 

Further  evidence  in  favor  of  the  chromosomal  hy- 
pothesis is  found  in  certain  cases  of  hybrids  between 


114 


THE    CHROMOSOMES 


species  of  sea  urchins.  The  best  analyzed  cases  are 
those  that  Baltzer  has  worked  out.  Crosses  were 
made  between  four  species  of  sea  urchins;  one  such 
cross  will  serve  as  an  example  (Fig.  39).  The  eggs 
of  Sphaerechinus  were  fertilized  by  the  sperm  of 
Strongylocentrotus.  The  division  of  the  chromo- 
somes proceeded  in  normal  manner.  The  pluteus 


FIG.  39. — 1  and  la,  chromosomes  in  the  first  normal  cleavage  spindle  of 
Sphserechinus;  2,  equatorial  plate  of  two-cell  stage  of  same;  3  and  3a, 
spindles  of  two-cell  stage  of  egg  of  hybrid  of  Sphaerechinus  by  Strongy- 
locentrotus ;  4  and  4a,  same,  equatorial  plates ;  5  and  5a,  hybrid  of  Strongy- 
locentrotus by  Sphaerechinus  cleavage  spindle  in  telophase ;  6,  next  stage 
of  last;  7,  same,  two-cell  stage;  8,  same,  later;  9,  same,  four-cell  stage; 
10,  same,  equatorial  plate  in  two-cell  stage  (22  chromosomes);  11,  same, 
from  later  stage,  24  chromosomes.  (After  Baltzer.) 

that  developed  was  intermediate  in  character;  or  at 
least  showed  peculiarities  both  of  the  maternal  and 
of  the  paternal  types.  The  reciprocal  cross  was  made 
by  fertilizing  the  eggs  of  Strongylocentrotus  with  the 
sperm  of  Sphaerechinus.  At  the  first  cleavage  of  the 
egg  some  of  the  chromosomes  divide  normally,  while 
other  chromosomes  remain  inactive  and  finally  be- 


THE    CHROMOSOMES  115 

come  scattered  in  the  region  between  the  others  that 
have  retreated  toward  the  poles.  When  the  division 
is  completed  the  belated  chromosomes  are  found  to 
be  excluded  from  the  daughter  nuclei.  They  appear 
irregular  in  shape  and  show  signs  of  degeneration. 
At  the  next  division  of  the  egg  they  may  still  be  found, 
but  they  are  lost  later,  and  seem  to  take  no  part  in  the 
development.  The  difference  between  this  and  the 
other  cross  seems  directly  caused  by  the  differences 
observed  in  the  behavior  of  the  chromosomes. 

A  count  of  the  chromosomes  in  the  hybrid  embryos 
shows  about  twenty-one  chromosomes.  The  mater- 
nal nucleus  contained  eighteen.  It  appears  that  only 
three  of  the  paternal  chromosomes  have  taken  a 
regular  part  in  the  development — fifteen  of  them  must 
have  degenerated  in  the  way  described  above.  The 
hybrid  embryos  that  developed  were  often  abnormal; 
the  few  that  developed  as  far  as  plutei  were  apparently 
entirely  maternal  in  character.  Since  the  reciprocal 
cross  proves  that  the  maternal  characters  are  not 
dominant,  the  most  reasonable  interpretation  is  that, 
although  the  foreign  sperm  had  started  the  develop- 
ment, it  had  produced  little  or  no  effect  on  the  char- 
acter of  the  larvae,  and  this  absence  of  effect  would 
seem  most  probably  to  be  due  to  the  elimination  of 
most  of  the  paternal  chromosomes.  It  might  pos- 
sibly be  maintained  that  the  same  kind  of  effect  pro- 
duced by  the  egg  of  Strongylocentrotus  on  the  chro- 
mosomes of  SphaBrechinus  is  likewise  produced  on  the 
protoplasm  introduced  by  the  sperm.  But  there  is 
here,  in  contrast  to  the  case  for  the  chromosomes. 


116  THE    CHROMOSOMES 

no  evidence  of  any  abnormal  cytoplasmic  behavior 
which  could  account  for  the  observed  abnormal  effect. 

Tennent  also  has  found  that  when  the  sea  urchin 
Toxopneustes  (?)  is  crossed  to  Hipponoe  (<?)  no 
loss  of  chromatin  occurs,  and  the  larvae  are  predomi- 
nantly paternal,  but  in  the  reciprocal  cross  (Hipponoe 
9  by  Toxopneustes  <?)  some  of  the  chromatin  is 
eliminated  and  the  larvae  are  more  like  the  maternal 
type. 

Some  experiments  by  Herbst  also  have  an  impor- 
tant bearing  on  the  question.  The  eggs  of  Sphaere- 
chinus  were  put  into  sea  water  to  which  a  little 
valerianic  acid  had  been  added.  This  is  one  of  the 
recognized  methods  of  starting  parthenogenetic  de- 
velopment. After  five  minutes  the  eggs  were  taken 
out  and  put  into  pure  sea  water  to  which  sperm  of 
Strongylocentrotus  was  added.  The  sperm  fertilized 
a  few  of  the  eggs.  The  eggs  had  already  begun  to 
undergo  some  of  the  changes  that  lead  to  develop- 
ment. The  belated  sperm  failed  to  keep  pace  with 
the  division  so  that  the  paternal  chromosomes  did 
not  reach  the  poles  of  the  egg  before  the  egg  chromo- 
somes reformed  their  nuclei  (Fig.  40).  In  conse- 
quence, the  paternal  chromosomes  formed  a  nucleus 
of  their  own  that  came  to  lie  in  one  of  the  cells  formed 
by  the  division  of  the  egg.  As  a  result  one  cell  had  a 
maternal  nucleus  and  the  other  had  a  double,  paternal 
and  maternal,  nucleus.  In  later  development  the 
paternal  nucleus  became  incorporated  with  the 
maternal  nucleus  of  its  cell.  Embryos  were  found 
later,  in  the  cultures,  that  were  on  one  side  maternal 


THE    CHROMOSOMES 


117 


and  on  the  other  side  hybrid  in  character  and 
probably  came  from  such  half -fertilized  eggs.  It 
will  be  recalled  that  Baltzer  has  shown  that  when 
the  cross  is  made  in  this  direction  both  paternal  and 


FIG.  40. — 1,  The  chromosomes  of  the  egg  lie  in  the  equator  of  the 
spindle,  the  chromosomes  of  the  sperm  at  one  side;  2,  a  later  stage  show- 
ing all  of  the  paternal  chromosomes  lying  at  one  side  passing  to  one  pole; 
3  (to  the  right), later  stage;the  conditions  are  the  same;  there  is  also  a 
supernumerary  sperm  in  the  egg  (shown  to  the  left,  in  another  section); 
4,  same  condition  as  last;  5,  pluteus  larva  that  is  purely  maternal  on  one 
side,  and  hybrid  on  the  other.  (After  Herbst.) 

maternal  chromosomes  behave  normally  at  each 
division.  The  conclusion  follows  with  much  plausi- 
bility that  the  absence  of  paternal  characters  on  one 
side  is  due  to  the  absence  of  paternal  chromosomes 
on  that  side. 


118  THE  CHROMOSOMES 

THE  INDIVIDUALITY  OF  THE  CHROMOSOMES 

The  view  that  the  chromosomes  are  persistent  as 
individual  structures  in  the  cell  has  steadily  gained 
ground  during  the  last  twenty  years.  The  process 
of  karyokinetic  or  mitotic  division  by  means  of  which 
at  each  cell  division  the  halves  derived  from  a  length- 
wise split  of  each  chromosome  are  carried  to  opposite 
poles,  so  that  a  genetic  continuity  is  maintained  be- 
tween corresponding  chromosomes  (and  parts  of 
chromosomes)  in  mother  and  daughter  cells,  has  been 
found  to  be  almost  universal  in  both  plants  and 
animals.  It  is  true  that  several  instances  have  been 
described  in  which  the  nucleus  simply  pinches  into 
two  parts,  and  there  can  be  little  doubt  that  such  cases 
occur;  but  no  one  has  been  able  to  show  in  a  convinc- 
ing way  that  cells  which  have  once  divided  in  this 
manner  ever  return  to  the  regular  process  of  karyo- 
kinetic division.  Case  after  case  of  amitosis  that 
has  been  described  for  the  germ  cells  has  been  either 
disproven,  or  found  to  rest  on  faulty  observation,  or 
else  to  relate  to  cells  like  those  of  the  egg  coats  that 
take  no  part  in  the  germinal  stream. 

There  are  several  observations  that  lead  to  the 
view,  at  present  generally  accepted,  that  the  chromo- 
somes retain  their  individuality  from  one  cell  division 
to  the  next.  These  may  now  be  given. 

During  the  resting  stage  the  chromosomes  spin  out 
in  such  a  way  that  they  appear  to  form  a  continuous 
network  in  the  nucleus.  They  can  not  be  identified 
individually  during  this  period.  When  the  chromo- 


THE    CHROMOSOMES 


119 


somes  again  become  visible,  preparatory  to  the  next 
division,  it  has  been  found  by  Boveri  in  Ascaris, 
which  is  particularly  well  suited  for  the  study  of  this 
point,  that  in  sister  cells  the  configuration  of  the 
groups  of  chromosomes  is  the  same  (Fig.  41).  The 
similarity  of  the  sister  cells  would  be  expected  had 
the  chromosomes  retained  during  the  resting  stage 
the  same  shape  and  size  and  relative  location  that 
they  had  at  the  end  of  the  last  division.  On  no  other 


.   *  c          * 

FIG.  41. — Four  pairs  of  sister  cells  of  Ascaris,  in  which  the  chromo- 
somes are  reappearing.  Note  the  similarity  of  arrangement  in  the  cells 
of  each  pair.  (After  Boveri.) 

view  can  we  so  readily  understand  the  similarities 
between  the  sister  cells;  for,  in  other  cells  of  these 
same  embryos  that  are  not  sister  cells,  a  great  variety 
of  arrangements  is  found,  and  no  two  arrangements 
are  so  nearly  alike  as  are  those  that  are  found  in 
cells  that  have  separated  from  each  other  at  the  last 
division.  In  a  few  instances  certain  observers  be- 
lieve that  they  have  even  been  able  to  distinguish 
the  separate  chromosomes  throughout  the  whole 


120  THE    CHROMOSOMES 

resting  period  of  the  cells,  but  this  must  be  received 
with  some  caution.  In  many  animals  and  in  some 
plants  the  chromosomes  are  of  very  different  sizes 
and  shapes,  and  many,  or  even  all  of  them,  can  be 
identified  at  each  division.  It  is  found  that  these  size 
relations  hold  throughout  all  divisions  of  the  cells. 
While  this  evidence  appears  at  first  sight  to  show 
that  the  chromosomes  are  structures  that  perpetuate 
themselves,  preserving  their  identity,  yet  it  might  be 
maintained,  in  fact  it  has  been  maintained,  that  each 
species  has  its  own  peculiar  protoplasm  from  which 
chromosomes  of  a  particular  kind  and  number  are, 
as  it  were,  crystallized  out  anew  before  each  cell 
division.  This  point  of  view  can  not,  however,  be  rec- 
onciled with  the  evidence  that  follows.  In  Meta- 
podius,  Wilson  has  found  that  individuals  may  differ 
in  the  particular  chromosome  that  he  calls  the  m 
chromosome.  While  the  normal  individuals  have  a 
pair  of  m  chromosomes,  one  individual  had  three 
m's;  but  all  of  the  cells  of  any  given  individual 
have  the  same  number.  These  chromosomes  furnish 
strong  support  of  the  continuity  of  the  chromosomes; 
for,  in  whatever  number  they  enter  the  individual 
during  fertilization,  they  retain  that  number  through- 
out all  the  subsequent  generations  of  cells.  The  same 
is  true,  of  course,  for  the  sex  chromosomes. 

Corroborative  proof  is  found  in  certain  hybrids, 
where  the  evidence  is  even  more  significant,  because 
in  such  cases  the  chromosomes  introduced  by  the 
male  are,  as  it  were,  in  a  foreign  medium.  For 
example,  Moenkhaus  first  pointed  out  that  when 


THE    CHROMOSOMES 


121 


the  fish  Fundulus  is  crossed  to  another  fish,  Menidia, 
the  two  kinds  of  chromosomes  present  in  the  fertilized 
egg  can  readily  be  distinguished  in  later  divisions. 
Similar  observations  have  been  made  for  many  other 
crosses  (Fig.  42)  by  Morris,  G.  and  P.  Hertwig, 
Federley,  Doncaster,  Rosenberg,  etc.  Despite  the 
fact  that  the  paternal  chromosomes  are  in  a  foreign 


ill 


a 


FIG.  42. — a,  Telophase,  division  of  an  embryonic  cell  of  Fundulus;  b, 
telophase,  division  of  an  embryonic  cell  of  egg  of  Fundulus  fertilized  by 
sperm  of  Ctenolabrus.  (After  Morris.) 

medium  they  retain  their  characteristic  size,  form, 
and  number.  The  embryos  from  these  eggs  are 
abnormal,  and  often  die,  not  because  chromosomes 
are  eliminated  but  because  the  combination  does  not 
work  out  successfully.  On  the  other  hand,  in  hybrid 
embryos  (studied  by  Herbst,  Baltzer,  and  Tennent) , 
in  which  paternal  chromosomes  are  eliminated,  they 


122  THE    CHROMOSOMES 

seem  never  to  re-appear  subsequently,  while  those 
not  eliminated  always  re-appear  at  the  next  cell 
division.  Other  cases  of  the  same  sort  are  known. 

In  general  it  may  be  said  that  even  an  abnormal  set 
of  chromosomes,  once  established  in  a  cell,  tends  to 
persist  through  all  succeeding  cell  generations.  This 
evidence  indicates  that  the  chromosomes  are  not 
mere  products  of  the  rest  of  the  cell  but  are  self- 
perpetuating  structures. 

THE  CHROMOSOMES  DURING  THE  MATURATION  OF 
THE  GERM  CELLS 

On  the  most  essential  point  concerning  the  matura- 
tion of  the  egg  and  sperm  there  is  no  dispute:  the 
observed  number  of  chromosomes  is  reduced  to  half. 
It  is  generally  agreed  that  this  lowering  of  the  number 
is  due  to  the  union  of  similar  chromosomes  in  pairs, 
each  chromosome  derived  from  the  father  conjugating 
with  the  homologous  chromosome  derived  from  the 
mother.  In  cases  where  different  chromosomes  can 
be  distinguished  by  their  shape  or  size  relations,  the 
relations  of  these  pairs  correspond  exactly  to  what 
they  should  be  if  like  chromosomes  conjugated. 

When  we  come  to  consider  how  this  union  of 
chromosomes  is  brought  about,  there  is  much  diver- 
gence of  opinion,  for  the  evidence  is  fragmentary  or 
contradictory  on  almost  every  point.  The  reason 
for  this  uncertainty  is  clear:  the  stages  at  which  the 
reduction  in  the  number  of  the  chromosomes  takes 
place  are  extraordinarily  difficult  to  interpret,  be- 


THE    CHROMOSOMES  123 

cause  at  this  time  the  chromosomes  are  in  the  form 
of  what  seems  to  be  a  dense  tangle  of  long  threads. 
When  this  stage  has  been  passed  through,  and  the 
chromosomes  'are  distinguishable  again,  the  pairing 
has  been  completed.  For  any  information  that  is 
worth  while  we  have  to  rely  on  the  best  material 
available.  It  may  be  disputed  which  material  is  the 
best,  but  it  will  be  generally  conceded  that  a  few 
types  have  shown  themselves  superior  to  others. 
The  account  of  maturation  that  is  here  followed 
confines  itself  to  two  types — one  for  the  male  and  the 
other  for  the  female.  These  are  selected  cases,  it  is 
true,  but  they  are  those  that  give,  in  the  opinion 
of  the  writers,  two  of  the  most  complete  accounts  of 
these  stages.  The  selection  is  admittedly  not  with- 
out bias,  for  these  types  can  be  most  advantageously 
utilized  to  illustrate  how  crossing  over  can  take 
place  between  the  members  of  homologous  pairs  of 
chromosomes. 

The  salamander,  Batracoseps  attenuatus,  has  fur- 
nished some  of  the  best  material  for  the  study  of  the 
ripening  of  the  germ  cells  of  the  male.  The  account 
that  follows  is  taken  from  Janssens'  elaborate  and 
detailed  study  of  the  spermatogenesis  of  Batracoseps. 

At  the  end  of  the  multiplication  period  (spermato- 
gonial  divisions)  the  nucleus  appears  as  shown  in 
Fig.  43,  a.  It  then  passes  into  a  condition  resembling 
a  resting  stage,  b.  Later  the  chromosomes  begin  to 
emerge  in  the  form  of  long  thin  threads  as  shown  in 
c,  d,  e.  In  the  last  figure  (the  leptotene  stage)  the 
ends  of  the  thin  threads  are  directed  toward  one  pole 


124 


THE    CHROMOSOMES 


where  some  of  the  ends  can  be  seen  to  be  arranged  in 
pairs.  As  they  unite  in  pairs  these  thin  threads 
often  have  the  appearance  of  twisting  tightly  around 


FIG.  43. — Spermatogenesis  of  Batracoseps  attenuatus.  a,  late  telophase 
of  spermatogonial  division ;  b,  resting  stage  after  the  last  spermatogonial 
division;  c,  appearance  of  the  spireme;  d  and  e,  later  stage  of  last  (bouquet 
grele) ;  /,  g,  h,  twisting  of  leptotene  threads  around  each  other  (amphitene 
stage);  i,  amphitene  stage  (entire  cell);  j,  pachytene  stage  (bouquet 
pachytene);  fc,  longitudinal  splitting  of  threads  (strepsinema  stage);  I, 
shortening  and  thickening  of  the  chromosomes.  (After  Janssens.) 


THE    CHROMOSOMES  125 

each  other,  beginning  at  the  end  where  they  first 
approached  each  other.  The  details  of  the  union 
of  the  threads  are  further  shown  in  /,  g,  h.  As  they 
unite  they  contract  until  they  are  in  the  form  of  a 
thicker  thread,  as  seen  in  i,  where  the  process  of 
fusion  has  progressed  as  far  as  the  middle  of  the 
nucleus.  Later,  j,  the  threads  become  fused  through- 
out their  length  (pachytene  stage).  Still  later  the 
thick  threads  begin  to  show  a  longitudinal  split 
(diplotene  stage),  and  cross  connections,  uniting  the 
halves  of  the  threads,  appear  in  different  places. 
The  threads  thicken  until  finally  a  stage  is  reached 
like  that  shown  in  k,  which,  by  further  contraction, 
reaches  the  condition  shown  in  /,  a  stage  preparatory 
to  the  first  maturation  division.  The  threads  of 
each  pair,  in  all  the  stages  of  the  latter  part  of  the 
diplotene  stage,  are  much  twisted  around  each 
other;  they  are  now  so  thick  that  they  show  the 
twisted  condition  very  plainly. 

The  egg  undergoes  a  series  of  changes  during  its 
maturation  which  parallels  those  of  the  sperm,  and 
which  leads  also  to  the  reduction  in  the  number  of  the 
chromosomes  to  half  of  the  full  number.  The  eggs 
of  a  shark  (Pristiurus  melanostomus)  have  been 
described  by  Marechal  as  passing  through  the 
following  stages.  At  the  end  of  the  period  of  multi- 
plication the  eggs  pass  into  a  resting  stage  (Fig.  44,  a) 
in  which  the  chromatin  appears  as  a  delicate  reticu- 
lum.  A  later  stage  is  shown  in  b,  c,  when  the  separate 
thin  threads  begin  to  make  their  appearance,  and 
take  parallel  courses,  d  (leptotene  stage).  These 


126 


THE    CHROMOSOMES 


FIG.  44. — The    growth,    synapsis,  and  reduction  stages  in  the  egg  of 
Pristiurus  melanostomus.     (After  Mar6chal.) 


THE    CHROMOSOMES  127 

thin  threads  next  assume  the  form  of  loops  with  their 
free  ends  pointing  toward  one  pole,  e  (bouquet 
stage,  also  called  the  period  of  synapsis).  At  their 
free  ends  the  threads  soon  appear  to  meet  in  pairs, 
d  and  e.  Each  pair,  by  the  apparent  fusion  of  its 
threads,  leads  to  the  formation  of  a  thick  thread  in 
the  form  of  a  loop,  /.  Further  condensation  and 
separation  of  the  threads  leads  to  the  condition  shown 
in  g.  The  thick  double  threads  next  show  a  length- 
wise split,  the  halves  being  often  twisted  around 
each  other  (diplotene  stage)  h.  The  pairs  of  threads 
nowT  begin  again  to  become  longer  and  to  occupy 
more  of  the  interior  of  the  nucleus  as  seen  in  i.  The 
eggs  have  grown  larger  meanwrhile  and  the  yolk 
appears.  As  the  nucleus  grows  still  larger,  keeping 
pace  with  the  growth  of  the  cell,  the  chromosomes 
begin  to  lose  their  staining  capacity.  Despite  the 
difficulty  of  tracing  the  chromosomes  throughout  the 
remaining  period,  Marechal  has  succeeded  in  follow- 
ing them,  step  by  step.  His  drawings  of  the  chro- 
mosomes give  the  impression  of  the  existence  of  a 
central  core  or  filament  remaining,  as  shown  in 
Fig.  44  i,  j,  k.  Delicate  loops  and  threads  are 
attached  to  this  core  and  may  be  traced  out  into  the 
region  of  each  side  of  the  chromosome.  During 
these  stages  deeply  staining  balls  of  material,  the 
nucleoli,  appear  in  the  nucleus.  Finally  the  chro- 
matin  threads  begin  to  condense  again  and  once 
more  take  the  stain ;  the  chromosomes  are  found  lying 
in  pairs  often  twisted  around  each  other  as  before,  as 
seen  in  I.  They  pass  in  this  condition  on  to  the  first 


128 


THE    CHROMOSOMES 


polar  spindle,   which   develops  in   the   egg   as   the 
nuclear  membrane  breaks  down. 

At  the  time  when  the  double  chromosomes  of  the 
sperm  and  the  egg  are  about  to  pass  onto  the  first 


FIG.  45. — Diagram  to  illustrate  the  two  reduction  divisions  of  sperm- 
atogonial  cells,  a,  first  spermatocyte  with  two  tetrads;  6  and  c,  division 
of  last;  d  and  /,  division  of  two  cells  of  c;  e  and  g,  completion  of  second 
division. 

maturation  spindle  each  half  of  the  double  chromo- 
some splits  lengthwise  so  that  four  parallel  strands 
are  present  (Figs.  45  and  46) ;  such  a  group  of  strands  is 
known  as  a  tetrad.  It  is  usually  held,  although  there 


THE    CHROMOSOMES  129 

is  some  dissent,  that  the  first  longitudinal  split  that 
appears  in  the  thick  thread  (pachytene  stage)  lies 
between  the  two  chromosomes  that  had  previously 
come  together,  such  a  separation  of  the  members  of  a 
pair  of  chromosomes  being  known  as  a  reductional 
split.  The  second  lengthwise  split  is  supposed  to 
separate  like  halves  of  the  same  chromosomes.  It  is 
called  an  equational  split. 

These  two  splits  are  in  preparation  for  the  two 
maturation  divisions  that  usually  take  place  in  rapid 
succession,  without  an  intervening  resting  stage. 
It  is  customary  therefore  to  look  upon  the  second 
lengthwise  split  as  a  precocious  split  in  the  chromo- 
somes preparatory  to  the  second  division.  If  the 
reduction  in  the  number  of  the  chromosomes  to  half 
of  the  original  number  were  the  sole  object  of  the 
reduction  divisions,  one  division  would  suffice  to 
separate  the  two  chromosomes  of  a  pair  that  had 
united  and  it  is  not  apparent  why  there  should  be  a 
second  division  at  all. 

The  two  maturation  divisions  with  tetrad  forma- 
tion are  typically  illustrated  in  the  changes  that  take 
place  in  the  spermatogenesis  and  oogenesis  of 
Ascaris,  the  thread  worm  of  the  horse,  as  worked  out 
by  van  Beneden,  Brauer,  0.  Hertwig  and  others. 
In  one  variety  four  chromosomes  occur  which  become 
reduced  to  two;  hence  there  are  only  two  tetrads 
present  (Fig.  45,  a).  At  the  first  division  two  halves 
of  each  thread  move  to  one  pole  and  two  to  the 
other  as  in  b  and  c.  At  the  second  division  the 
separation  of  the  two  remaining  threads  takes  place, 


130 


THE    CHROMOSOMES 


d  and  /.  At  the  end  of  the  process  there  are  two 
chromosomes  remaining  in  each  of  the  four  cells,  e 
and  g.  Each  cell  becomes  a  spermatozoon.  Here 
as  in  most  cases  there  is  nothing  to  show  whether 
the  first  division  is  reductional  and  the  second 
equational,  or  the  reverse.  There  is  much  divergence 
of  opinion  on  this  point  for  different  species.  The  end 


d  e  f 

FIG.  46. — Diagram  to  show  the  extrusion  of  the  two  polar  bodies. 
Two  tetrads  are  represented  in  a.  The  two  succeeding  divisions  b-c, 
d-e,  show  the  separation  of  the  members  of  the  tetrads  with  the  result 
that  one  of  each  kind  is  left  in  the  egg. 

result,  however,  is  the  same  so  far  as  the  genetic 
problem  is  concerned,  the  sequence  being  ordinarily 
a  matter  of  no  significance. 

In  the  egg  (Fig.  46)  the  process  is  identical  with 
that  in  the  sperm,  except  that  one  of  the  two  cells 
formed  is  much  smaller  than  the  other.  The  small 
cell  is  the  polar  body.  At  the  first  division  the  nucleus 


THE    CHROMOSOMES  131 

sends  out  half  of  its  chromatin  into  the  first  polar 
body  (Fig.  46,  c).  Without  a  resting  stage  a  new 
spindle  is  formed  around  the  chromosomes  in  the  egg 
and  a  second  polar  body  is  thrown  off,  as  in  e.  The 
first  polar  body  may  also  divide.  The  three  polar 
bodies  and  the  egg,  /,  are  comparable  to  the  four 
spermatozoa.  All  four  spermatozoa  are  functional, 
but  only  one  product  of  the  two  divisions  of  the  egg 
is  functional.  Unless  the  tetrad  is  specifically 
oriented  upon  the  polar  spindle  of  the  egg  the  chance 
is  equally  good  that  any  one  of  the  four  threads  that 
make  up  the  tetrad  will  be  the  one  that  remains  in 
the  egg. 

CROSSING  OVER 

If  the  preceding  account  of  the  maturation  of  the 
egg  and  of  the  sperm  were  accepted  as  covering  the 
entire  behavior  of  the  chromosomes  during  this 
period,  there  would  be  no  possibility  for  an  interchange 
between  the  members  of  a  pair.  But  there  are 
several  stages  in  the  ripening  of  the  germ  cells  when 
an  interchange  between  homologous  chromosomes 
might  possibly  take  place.  For  instance,  when  the 
thin  threads  are  coming  together  (Fig.  43,  e,  f,  g,  h) 
several  observers  have  described  them  as  twisting 
around  each  other  (synaptic  twisting)  as  represented 
in  these  figures.  If  where  the  threads  cross  a  part  of 
one  thread  becomes  continuous  with  the  remainder  of 
the  other  thread  (Fig.  24)  an  interchange  of  pieces 
•will  have  been  accomplished.  If,  as  shown  in  Fig. 
24,  B,  the  chromosomes  are  represented  as  a  linear 


132  THE    CHROMOSOMES 

series  of  beads  (chromomeres),  then,  when  the  con- 
jugating chromosomes  twist  around  each  other, 
whole  sections  of  one  chain  will  come  to  lie,  now  on 
one  side,  now  on  the  other  side,  in  the  double  chro- 
mosome. If,  when  the  two  series  of  beads  come  to 
separate  from  each  other,  all  of  the  segments  that  lie 
on  the  same  side  tend  to  go  to  one  pole,  and  all  of 
those  on  the  opposite  side  to  the  other  pole,  each 
series  must,  in  order  to  separate,  break  apart  between 
the  beads  at  the  crossing  point.  Moreover,  since 
the  essential  part  of  the  process  is  that  homologous 
beads  go  to  opposite  poles  it  follows  that  the  break 
between  the  beads  of  two  chains  must  always  be  at 
identical  levels.  It  is  not  necessary  to  assume  that 
crossing  over  takes  place  at  every  node,  but  only  that 
it  may  sometimes  take  place.  In  fact,  our  work  on 
Drosophila  shows  for  the  sex  chromosome  in  the 
female  that  crossing  over  takes  place  in  only  about 
half  of  the  cells,  and  double  crossing  over  is  a  rather 
rare  event. 

There  is  a  later  stage  also  at  which  crossing  over 
might  be  supposed  to  take  place.  After  the  thin 
threads  have  conjugated  to  form  the  thick  threads, 
and  these  have  shortened  and  split  lengthwise,  four 
strands  are  present  (Fig.  47).  If  two  of  the  strands 
fuse  at  the  crossing  place  (the  pieces  of  one  strand 
uniting  endwise  with  the  pieces  of  the  other)  crossing 
over  is  brought  about.  It  is  this  type  in  particular 
that  Janssens  named  chiasmatype.  In  support  of 
this  method  of  crossing  over  are  Janssens'  observa- 
tions on  Batracoseps,  where  he  concludes  from  the 


THE    CHROMOSOMES  133 

method  by  which  the  strands  are  found  joined  at  the 
time  when  they  draw  apart,  that  cross  union  of  the 
threads  must  have  previously  taken  place. 

If  crossing  over  be  supposed  to  take  place  between 
two  single  threads  (Fig.  24)  all  four  gametes  that 
ultimately  result  from  such  a  cell  will  be  crossover 
gametes.  On  the  other  hand,  if  crossing  over  takes 


c  D 

FIG.  47. — Four  stages  in  crossing  over,  according  to  the  "typical" 
chiasma  type  of  Janssens.  The  white  rod  and  the  black  rod  are  each 
split  lengthwise;  crossing  over  takes  place  only  between  two  of  the  four 
strands. 

place  by  means  of  the  chiasmatype  (Fig.  47)  only 
two  of  the  resulting  four  cells  will  be  crossover 
gametes,  the  other  two  being  non-crossover  gametes.1 
Looked  at  from  the  point  of  view  of  the  total 
output,  there  would  be  no  way  in  which  to  tell 
whether  one  or  the  other  of  the  above  processes  has 
taken  place;  although  the  formation  of  a  given 
number  of  crossover  gametes  involves  only  half  as 
many  participating  cells  in  the  case  of  the  single 
thread  type  as  in  the  case  of  the  double  thread  type. 

1  If,  after  the  thick  threads  have  split,  crossing  over  involving  both 
strands  of  each  chromosome  should  take  place,  instead  of  only  one 
strand  as  in  the  chiasmatype,  sensu  strictu,  the  four  gametes  that  result 
would  be  crossover  gametes. 


134  THE    CHROMOSOMES 

At  present  it  seems  better  not  to  attempt  to 
commit  the  theory  of  crossing  over  to  one  rather  than 
to  another  of  these  stages;  for,  whether  the  process 
occurs  at  the  leptotene  thread  stage  as  suggested 
above,  or,  as  Janssens  believes,  at  a  later  stage 
(strepsinema) ,  the  genetic  result  is  the  same.  What 
we  wish  to  point  out  is  that  in  the  phases  through 
which  the  chromosomes  pass  at  the  maturation 
stages  there  is  given  an  opportunity  for  an  inter- 
change of  parts.  The  genetic  evidence  shows  very 
clearly  that  interchanges  do  take  place,  as  is  best 
illustrated  in  the  case  of  the  sex  chromosomes, 
whose  history  can  be  traced  with  some  assurance 
from  one  generation  to  the  next. 

What  we  wish  especially  to  insist  upon  and  empha- 
size is  that  the  evidence  from  linkage  in  Drosophila 
has  shown  beyond  any  doubt  that  crossing  over  is 
not  a  process  that  involves  only  a  particular  factor 
in  relation  to  its  allelomorph.  Our  work  has  shown 
positively  that  there  is  a  tendency  for  large  sections 
of  the  chromosomes  to  interchange  whenever  crossing 
over  occurs. 

Another  idea  that  is  likely  to  suggest  itself  in  this 
connection  has  also  been  disproven  by  the  evidence 
from  Drosophila.  It  might  be  supposed  that  at  a 
resting  stage  the  chromosomes  go  to  pieces  and  the 
fragments  come  together  again  before  the  next 
division  period.  Linkage  might  then  mean  the 
likelihood  of  fragments  remaining  intact,  etc.  But 
if  the  chromosomes  broke  up  completely  into  their 
constituent  elements  at  each  resting  period  then 


THE    CHROMOSOMES  135 

there  is  no  explanation  as  to  why  the  factors  in  a 
group  remain  together  in  sections  as  explained  on 
page  66.  If  it  is  supposed  that  the  chromosomes 
break  only  once  or  twice,  and  that  linkage  represents 
the  holding  together  of  the  pieces,  then  one  is  forced 
to  assume  that  the  breaking  up  is  the  same  in  both 
members  of  a  pair,  yet  entirely  inconstant  in  different 
cells;  for  otherwise  the  reunion  of  the  fragments 
would  lead  to  duplication  or  loss  of  whole  sections 
of  the  chromosomes,  and  all  order  would  soon  be  lost. 
A  large  amount  of  data  relating  to  sex  linked  char- 
acters has  shown  that  the  sex  chromosomes  must 
remain  intact  as  often  as  they  break  apart,  and  even 
when  they  break  apart  this  takes  place,  as  a  rule,  at 
only  one  place. 


The  interpretation  of  Mendelian  inheritance  on  a 
chromosomal  basis  by  no  means  excludes  the  possi- 
bility that  there  may  be  other  forms  of  inheritance 
depending  on  other  cell  materials.  Although  the 
cytoplasm  is  essential  for  the  development  of  the 
organism,  and  is  transmitted  by  the  egg  to  each  new 
generation,  its  materials  do  not  perpetuate  themselves 
unchanged  as  do  the  chromosomes,  and  are  therefore 
really  not  hereditary.  There  are,  however,  certain 
bodies  carried  by  the  protoplasm,  such  as  plastids 
(possibly  also  chondriosomes) ,  which  like  the  chro- 
matin  are  able  to  grow  and  divide,  and  hence  might 
have  the  power  to  peTpetuate  themselves  unchanged 


136  THE    CHROMOSOMES 

indefinitely.  Such  bodies  might  not  only  produce 
passive  products,  like  starch  or  pigment,  but  even 
active  enzymes,  which,  interacting  with  other 
products  of  development,  might  determine  the 
characteristics  of  the  race. 

Structures  like  the  shell  and  the  yolk  of  eggs  are 
purely  maternal  in  origin,  but  since  they  do  not  have 
the  power  of  growth  and  division,  they  are  not  able 
to  perpetuate  themselves  indefinitely,  nevertheless 
they  may  determine  certain  characteristics  of  the 
embryo,  and  to  this  extent  may  appear  to  influence 
the  hereditary  characters  of  the  generation  to  which 
the  embryo  belongs.  For  instance,  the  females  of 
certain  races  of  silkworm  moths  have  white  eggs,  be- 
cause the  shell  is  white.  If  such  eggs  are  fertilized  by 
sperm  of  another  race,  that  has  eggs  with  a  domi- 
nant green  colored  shell,  the  shells  are  nevertheless 
white.  Conversely  when  the  green  eggs  of  a  female 
moth  of  the  green  egg  race  are  fertilized  by  the 
sperm  of  a  male  of  a  white  egg  race,  the  color  re- 
mains green.  When  the  moths  develop  from  either 
of  these  two  kinds  of  hybrid  eggs,  one  white,  one 
green,  they  lay  only  green  eggs,  because  in  the  hybrid 
the  factor  for  green  dominates  and  determines  the 
color  of  the  shell  that  is  produced  in  the  new  eggs. 
These  green  eggs  give  rise  to  moths,  three  of  which 
lay  eggs  that  are  green  to  one  that  lays  eggs  that  are 
white,  showing  that  here  there  is  only  the  ordinary 
case  of  Mendelian  inheritance,  which  is  obscured, 
however,  when  the  characters  of  the  young  embryo 
are  considered,  because,  as  has  been  shown,  these 


THE    CHROMOSOMES  137 

characters  are  due  to  peculiarities  of  the  eggs  before 
they  are  laid. 

The  serosa  on  the  other  hand  is  a  cellular  membrane 
that  develops  around  the  embryo  and  produces  pig- 
ment. The  pigment  seen  through  the  shell  gives  the 
embryo  a  definite  color,  which  in  the  hybrid  embryo 
is  characteristic  of  the  maternal  race.  Since  the 
serosa  pigment  is  not  present  in  the  egg,  but  develops 
after  fertilization  the  inheritance  here  appears  to  be 
determined  by  the  character  of  the  egg  and  not  by  the 
sperm.  But  the  genetic  history  of  this  character  of 
the  embryo  is  apparently  the  same  as  that  of  the 
color  of  the  shell  or  of  the  yolk.  It  can,  therefore,  be 
interpreted  in  the  same  way.  There  must,  then, 
be  present  in  the  egg  some  substance  that  is  at  first 
uncolored,  and  later  this  substance  when  carried  into 
the  serosa  produces  pigment,  presumably  by  inter- 
acting with  something  else  there.  In  the  next  genera- 
tion, however,  the  influence  of  the  father  comes  to 
light  when  the  F2  embryo  produces  its  serosa  mate- 
rial; for  now  the  nucleus  of  the  PI  male  has  had  op- 
portunity to  determine  what  this  material  may  be, 
and  should  the  paternal  factor  be  the  dominant  one 
it  determines  the  kind  of  material  that  the  eggs  will 
contain  and  hence  the  color  of  the  serosa  of  this  new 
generation. 

A  case  of  cytoplasmic  inheritance  has  been  de- 
scribed by  Correns  in  the  four-o'clock,  Mirabilis 
jalapa.  There  is  a  race  whose  leaves  are  checkered 
with  green  and  white,  but  some  branches  may  have 
leaves  entirely  green,  other  branches  may  have  only 


138  THE    CHROMOSOMES 

white  leaves.  If  the  flowers  of  the  green  branches  are 
self-fertilized,  the  young  plants  are  green.  If  the 
flowers  of  the  white  branches  are  self-fertilized,  the 
offspring  have  white  leaves  and  these  plants  perish 
for  want  of  chlorophyll.  From  the  checkered 
branches  the  offspring  may  be  green,  or  checkered, 
or  white. 

When  a  cross  is  made  between  the  flowers  borne 
by  branches  that  are  unlike,  the  inheritance  is  purely 
maternal.  For  example,  if  the  pistil  of  a  white 
branch  is  fertilized  with  pollen  from  a  pure  green 
plant,  only  white  leaved  offspring  are  produced. 
The  reciprocal  cross,  the  pistil  from  a  green  branch 
fertilized  with  pollen  from  a  white  branch,  gives 
only  green  offspring,  and  these  remain  green  through 
all  subsequent  generations. 

Correns  points  out  that  these  results  can  be  inter- 
preted if  the  whitening  is  due  to  a  sort  of  disease  that 
is  carried  by  the  cytoplasm.  The  egg  cytoplasm 
carries  over  the  disease  to  the  next  generation.  As 
the  pollen  does  not  bring  in  any  cytoplasm  the 
disease  is  not  transmitted  through  the  male  side. 

Baur  points  out  that  in  several  other  plants  in 
which  varieties  with  leaves  marked  with  white  exist, 
as  in  Melandrium,  Antirrhinum,  etc.,  the  inheritance 
is  strictly  Mendelian,  for  the  Fi  generation  is  green 
and  the  F2  generation  is  made  up  of  three  greens  to 
one  marked  with  white.  In  these  cases  the  color  may 
depend  on  a  chromosomal  factor.  But  there  is  a 
case  in  Pelargonium  that  Baur  thinks  can  not  be 
explained  in  either  of  the  foregoing  ways.  Here 


THE    CHROMOSOMES  139 

again  there  are  mosaic  branches,  white  branches,  and 
also  green  branches.  Flowers  on  green  branches 
crossed  with  flowers  on  white  branches  give  mosaic 
plants,  irrespective  of  which  way  the  cross  is  made. 
A  self-fertilized  flower  from  a  green  branch  gives  rise 
to  a  plant  with  purely  green  leaves.  If  a  flower  from 
a  checkered  branch  is  self-fertilized  it  produces  a 
checkered  plant.  If  a  flower  from  a  white  branch  is 
self-fertilized  it  gives  rise  to  a  white  plant. 

Baur  suggests  tentatively,  the  following  hypothesis 
to  explain  the  case  of  Pelargonium.  The  green  color 
of  this  plant,  like  that  of  all  flowering  plants,  is  due 
to  chlorophyll  grains  and  these  grains  multiply, 
supplying  all  the  cells  in  generations  that  subse- 
quently arise  with  their  quota  of  grains.  In  the 
white  parts  these  grains  are  defective  in  the  sense 
that  they  fail  to  produce  the  green  color,  but  retain 
their  power  of  multiplying.  If  now  it  is  assumed 
that  the  pollen  as  well  as  the  egg  may  transmit  some 
chlorophyll  grains  the  results  can  be  explained.  For, 
in  the  division  of  the  cells  that  contain  both  green 
(normal)  and  white  (abnormal)  grains  there  will 
arise  at  times  an  unequal  distribution  of  the  grains, 
and  in  extreme  cases  two  kinds  of  branches  may  arise, 
one  with  green  and  the  other  with  white  grains.  The 
hypothesis  calls  for  transmission  through  the  cyto- 
plasm of  the  pollen  as  well  as  through  that  of  the 
egg  cell.  Baur  states  that  until  this  fact  can  be 
established  the  interpretation  must  be  uncertain. 


CHAPTER  VI 

THE    CORRESPONDENCE    BETWEEN    THE 

DISTRIBUTION  OF  THE  CHROMOSOMES 

AND  OF  THE  GENETIC  FACTORS 

Attention  has  been  called  to  the  fact  that  paired 
factors  are  distributed  in  the  same  way  as  are 
homologous  chromosomes,  and  that  factors  which 
are  assorted  independently  are  distributed  in  the 
same  way  as  non-homologous  chromosomes.  In 
proof  of  the  latter  point  there  is  Wilson's  evidence 
for  a  Metapodius  with  three  homologous  m-chromo- 
somes.  It  was  found  that  the  extra  m  goes  to  the 
gamete  that  receives  X  as  often  as  to  the  other 
gamete.  Miss  Carothers  describes  a  somewhat 
similar  case  in  certain  grasshoppers,  in  which  the 
distribution  of  a  pair  of  unequal  chromosomes  is 
independent  of  the  distribution  of  the  X  chromo- 
some. Not  only  are  the  pairs  of  factors  assorted 
independently,  as  are  the  chromosomes,  but  in 
Drosophila,  where  the  number  of  independently 
assorting  groups  of  factors  has  been  determined,  it 
has  been  found  that  the  number  is  identical  with  the 
number  of  chromosome  pairs.  Moreover,  even  the 
relative  sizes  of  the  groups — both  as  determined  by 
the  number  of  factors  they  contain  and  by  the  fre- 
quency of  crossing  over  within  them — are  the  same 

140 


DISTRIBUTION    OF    THE    CHROMOSOMES  141 

as  those  of  the  chromosomes.  Finally,  the  distribu- 
tion of  the  factors  within  any  one  group  is  what  the 
chromosome  hypothesis  calls  for.  For  the  fre- 
quencies of  separation  (or  combination)  between  the 
different  factors  of  a  group  are  in  a  linear  relation  to 
each  other,  and  the  relation  is  even  specifically  of 
such  a  type  (involving  interference)  as  would  be 
expected  to  occur  if  the  separations  between  the 
factors  resulted  from  the  crossing  over  between  two 
twisted  chromosomes  which  the  cytological  evidence 
indicates  may  occur. 

Even  in  cases  where  the  chromosomes  are  not 
distributed  in  the  usual  way  it  is  found  that  the 
factors  have  the  same  unusual  method  of  distribu-^ 
tion.  For  example,  in  moths  there  are  some  cases  of 
extraordinary  interest  because  the  chromosomes  can 
be  traced  to  and  through  the  ripening  period  of  the 
eggs  of  the  hybrid.  Certain  species  of  the  moth 
Pygsera  that  have  different  numbers  of  chromosomes 
were  crossed  by  Federley.  The  full  number  (calcu- 
lated) and  the  reduced  number  of  chromosomes  in 
the  different  species  are  as  follows: 

Diploid  Haploid 

P.  anachoreta                                 60  30 

P.  curtula                                       58  29 

P.  pigra                                            46  23 

In  the  hybrids,  the  full  number  is  the  sum  of  the  two 
haploid  sets  that  went  in  from  the  parents.  This 
shows  that  the  chromosomes  preserve  their  individ- 
uality through  many  successive  cell  divisions  in  a 


142  DISTRIBUTION    OF   THE    CHROMOSOMES 

foreign  cytoplasm.  In  the  maturation  a  few  of  the 
chromosomes  seem  at  times  to  unite  in  pairs,  but 
most  of  them  fail  to  do  so,  so  that  while  the  number 
of  the  chromosomes  at  the  first  maturation  division 
is  slightly  less  than  the  full  number  it  is  much  more 
than  half  of  that  number.  Different  types  of 
hybrids  behave  slightly  differently  in  respect  to  the 
extent  to  which  union  in  pairs  takes  place.  The 
failure  to  unite  indicates  that  in  normal  maturation 
homologous  chromosomes  mate  with  each  other, 
for  here  there  are  few  or  no  chromosomes  that  are 
strictly  homologous  and  yet  there  is  just  as  much 
opportunity  as  in  normal  maturation  for  non- 
homologous  chromosomes  from  the  same  parent  to 
unite. 

When  the  first  spermatocyte  division  takes  place 
in  the  hybrid,  all  the  unmated  chromosomes  divide, 
but  the  fewr  chromosomes  that  are  mated  pre- 
sumably separate.  Consequently  each  of  the  daugh- 
ter cells  has  the  double  number  of  chromosomes 
(a  set  from  each  parent  species),  except  for  the  few 
chromosomes  that  had  been  united  in  pairs.  At 
the  second  maturation  division  the  chromosomes 
again  divide,  so  that  the  spermatozoa  too  should 
receive  nearly  the  double  number  of  chromosomes, 
one  set  from  one  species,  the  other  set  from  the  other 
species. 

If,  then,  the  factors  are  contained  in  the  chromo- 
somes, we  should  expect  that,  except  for  any  factors 
in  the  few  chromosomes  that  mate  and  separate,  the 
hybrid  would  transmit  to  all  its  offspring  the  same 


DISTRIBUTION    OF    THE    CHROMOSOMES  143 

factors,  since  every  spermatozoon  receives,  with  the 
above  exceptions,  all  the  chromosomes  (paternal  and 
maternal)  that  the  hybrid  contains.  On  crossing  the 
hybrid  to  either  parent,  it  is  found  that  the  offspring 
actually  are  very  much  alike,  i.e.,  have  all  received 
practically  the  same  factors — a  striking  contrast 
to  the  result  usually  obtained  in  "backcrosses." 
In  respect  to  just  one  character  (a  larval  marking), 
however,  the  above  relation  does  not  hold,  but  ordi- 
nary Mendelian  results  are  obtained,  and  this  in  turn 
corresponds  with  the  fact  that  a  few  chromosomes 
do  undergo  segregation.  In  regard  to  the  other  char- 
acters, not  only  are  the  offspring  like  each  other,  but 
they  resemble  the  hybrid  more  than  either  of  the  pure 
species,  corresponding  with  the  fact  that  they  contain 
complete  sets  of  chromosomes  from  both  types.  But 
they  do  not  look  just  like  the  FI  hybrid,  and  cor- 
respondingly one  set  of  chromosomes  is  in  the  diploid, 
the  other  in  the  haploid  number.  This  is  because  they 
receive  a  set  of  one  species  from  both  parents,  but  a 
set  of  the  other  species  only  from  the  hybrid  parent. 
Federley  also  shows  that  when  maturation  takes  place 
in  this  triploid  individual  one  set  of  chromosomes 
does  not  undergo  mating,  but  the  others — presumably 
those  in  the  two  identical  sets — do  pair  with  each 
other,  so  that  the  total  number  is  reduced  to  one  bi- 
valent set,  and  one  single  set.  If  the  paired  chromo- 
somes separate  and  the  unpaired  ones  divide,  as  oc- 
curs in  the  FI  hybrid,  the  double  number  of  chromo- 
somes, a  set  of  each  species,  will  again  be  found  in  the 
sperm,  as  was  the  case  in  the  first  hybrid.  In  other 


144  DISTRIBUTION    OF    THE    CHROMOSOMES 

words  there  is  expected  no  return  to  either  parent 
type,  but  the  hybrid  when  backcrossed  always  con- 
tinues to  produce  hybrids.  Moreover,  there  is  no 
apparent  weakening  or  other  influence  exerted  by  the 
egg  on  the  foreign  chromosomes  even  in  successive 
generations.  The  breeding  results  of  Standfuss, 
who  backcrossed  other  moths  for  several  generations, 
show  exactly  this  phenomenon — the  same  type  of 
hybrid  constantly  produced  in  every  generation. 

A  similar  behavior  of  the  chromosomes  has  been 
recently  described  by  Doncaster  in  a  cross  between 
other  species  of  moths,  and  is  illustrated  in  the 
following  figures.  The  full  number  of  chromosomes 
in  the  moth  Biston  hirtaria  is  shown  in  Fig.  48,  a. 
There  are  28  in  all,  of  which  four  are  small.  Another 
species,  Biston  zonaria,  has  something  over  a  hundred 
very  small  chromosomes  (Fig.  48,6).  The  reduced 
number  of  chromosomes  of  the  former  species  is  13 
(one  large  one  being  coupled  with  a  small  one),  of 
the  latter  56.  The  chromosome  group  of  the  hybrid 
(zonaria  ?  by  hirtaria  «")  is  shown  in  Fig.  48,  c.  The 
exact  number  of  chromosomes  is  difficult  to  count,  but 
there  are  14  large  ones  and  about  56  small  ones.  In 
this  hybrid  a  stage  is  passed  through  that  resembles 
the  synapsis  stage.  When  the  chromosomes  emerge 
from  this  stage  (Fig.  48,  c') ,  almost  the  full  number  are 
found  present,  although  Doncaster  thinks  that  a  few 
of  them  have  united  in  pairs;  for  as  shown  in  the 
figure  there  are  now  12  or  13  large  and  50  or  51  small 
chromosomes.  These  are  a  few  less  than  the  full 
number  present  before  synapsis.  In  this  case,  how- 


DISTRIBUTION    OF   THE    CHROMOSOMES  145 

f 


*»**• 

*       -  * 


a  a' 


'•*•/* 


FIG.  48. — Biston  hirtaria;  a,  spermatogonial  chromosomes;  a',  primary 
spermatocyte  chromosomes  (reduced  number).  Biston  zonaria;  6,  sper- 
matogonial chromosomes;  b',  primary  spermatocytes  (reduced  number). 
Hybrid,  out  of  zonaria  female  by  hirtaria  male;  c,  spermatogonial  chro- 
mosomes; c',  primary  spermatocytes.  (After  Harrison  and  Doncaster.) 


146  DISTRIBUTION    OF   THE    CHROMOSOMES 

ever,  no  data  concerning  the  genetic  behavior  of  the 
hybrids  have  been  reported. 

Another  instance  of  parallelism  between  unusual 
chromosome  phenomena  and  genetic  results  is  that 
found  in  (Enothera  lata  and  semilata  by  Lutz,  Gates 
and  Thomas.  The  normal  chromosome  number  in 
(Enothera  lamarckiana  is  14,  but  the  race  called  lata 
always  has  15  chromosomes,  i.e.,  one  kind  of  chromo- 
some exists  in  the  triploid  number.  This  is  true 
even  of  lata  plants  which  originated  independently 
of  the  ordinary  stock,  in  widely  different  races  of 
(Enothera.  The  same  results  apply  to  semilata, 
which  appears  to  be  a  variety  of  lata.  Lata  and  semi- 
lata occasionally  arise  "  spontaneously  "  from  lamarck- 
iana, in  a  small  per  cent,  of  the  offspring  of  any  one 
individual,  and  the  explanation  for  this  may  be  found 
in  the  fact  that  occasionally,  in  the  gametogenesis 
of  lamarckiana,  two  mated  chromosomes,  instead  of 
separating,  pass  to  the  same  pole  (non-disjunction) 
so  that  the  offspring  would  have  three  chromosomes  of 
this  type  and  contain  15  chromosomes  in  all.  The 
behavior  of  the  extra  chromosome  in  the  lata  indi- 
viduals is  also  of  interest,  for  it  is  found  that  in 
gametogenesis,  when  the  mated  chromosomes  sepa- 
rate, the  extra  chromosome  does  not  divide  regularly 
as  do  unpaired  chromosomes  in  moths,  but  tends  to 
pass  to  one  pole.  This  would  result  in  half  the 
gametes  containing  it  and  transmitting  the  lata  con- 
dition and  the  other  half  being  normal.  Very  often, 
however,  the  chromosome  lags  on  the  spindle  and 
so  fails  to  be  included  in  the  nucleus  of  either  daughter 


DISTRIBUTION    OF    THE    CHROMOSOMES  147 

cell,  or  it  may  even  be  torn  apart,  as  if  by  spindle 
fibers  from  opposite  poles.  Consequently  less  than 
half  of  the  gametes  (at  least  the  sperm,  for  gameto- 
genesis  was  not  studied  in  the  female  organs)  receive 
the  extra  chromosome.  The  proportion  varies  greatly 
in  different  individuals.  This  conforms  with  the 
genetic  result  that  lata  individuals,  crossed  to  la- 
marckiana,  give  varying  proportions  of  lata  offspring 
but  never  produce  offspring  more  than  half  of  which 
are  lata. 

In  Primula,  a  striking  case  of  correspondence  be- 
tween abnormal  genetic  and  chromosome  phenomena 
has  been  found,  that  appears  strongly  in  favor  of  the 
chromosome  hypothesis,  although  the  discoverer, 
Gregory,  has  hesitated  to  draw  this  conclusion.  Two 
giant  races  of  the  primula  (P.  sinensis)  were  found  to 
have  twice  the  number  of  chromosomes  character- 
istic of  other  domesticated  races.  The  breeding  ex- 
periments with  these  plants  show  that  they  also  have 
a  double  set  of  factors  as  compared  with  the  same 
factors  in  ordinary  primulas.  While  in  ordinary  plants 
each  chromosome  is  double  and,  therefore,  each  factor 
is  represented  twice,  for  instance  by  A  and  A,  in  the 
giants  there  are  four  like  chromosomes,  hence  four 
factors  AAAA.  If  the  giant  race  contains  some  fac- 
tors already  mutated,  such  as  A1,  the  giant  might  con- 
tain one,  two,  three,  or  four  of  the  mutant  factors 
A1.  Such  plants  would  be  AAAA1  or  AAA^1  or 
AAWA1  or  A^A^AIA.1.  As  stated  above,  the  breed- 
ing work  shows  that  there  is  a  quadruple  set  of 
factors,  but  the  evidence  is  as  yet  insufficient  to  de- 


148  DISTRIBUTION    OF   THE    CHROMOSOMES 

cide  whether  a  mutant  factor  A1  has  as  its  mate 
(always  pairs  at  maturation  with)  a  special  one  of  the 
remaining  A's  or  may  become  the  mate  of  any  one  of 
the  three.  On  the  chromosome  hypothesis  we  should 
expect,  on  the  whole,  the  latter  to  be  true.  Which- 
ever of  these  views  becomes  established  the  parallel 
between  the  double  set  of  chromosomes  and  the 
double  set  of  factors  is  the  important  fact.  Gregory 
admits  this,  but  adds  the  caution:  "Yet  on  the  other 
hand  the  tetraploid  number  of  chromosomes  may  be 
nothing  more  than  an  index  of  the  quadruple  nature 
of  the  cell  as  a  whole." 

In  the  preceding  cases  it  has  been  shown  that  the 
factors  and  the  chromosomes  have  the  same  method 
of  distribution.  In  the  case  of  sex  and  sex  linked  fac- 
tors it  can  even  be  shown  that  they  have  the  same 
distribution  as  the  sex  chromosomes.  This  identity 
of  distribution  holds  not  only  for  F2  results  and  F3 
tests,  but  for  all  kinds  of  backcrosses  as  well.  The 
relation  holds,  moreover,  for  all  known  sex  linked 
factors,  of  which  in  Drosophila  there  are  more  than 
forty  cases,  and  for  all  combinations  of  sex  linked 
factors.  Not  to  interpret  this  evidence  to  mean 
that  the  factors  are  contained  in  and  carried  by  the 
chromosomes  is  to  reject  a  mechanistic  basis  known 
to  exist  in  the  cell.  Nothing  is  gained  if,  in  order  to 
avoid  the  obvious  connection  between  the  inheritance 
of  the  character  and  the  transmission  of  the  chromo- 
some, we  assume  that  something  else  in  the  cell,  a 
portion  of  the  cytoplasm,  perhaps,  also  follows  the 
distribution  of  the  sex  chromosomes.  Such  a  postu- 


DISTRIBUTION    OF    THE    CHROMOSOMES  149 

late  only  adds  an  unknown  and  improbable  assump- 
tion and  leaves  the  situation  less  clear  than  before. 

The  advantage  of  the  chromosomal  interpretation 
as  applied  to  the  sex  chromosomes  is  nowhere  better 
illustrated  than  in  the  history  of  a  process  called 
non-disjunction,  which  was  discovered  by  Bridges. 
Furthermore  this  case,  supported  on  the  one  hand 
by  extensive  and  definite  experimental  breeding  and 
on  the  other  hand  by  cytological  investigation,  offers 
the  most  direct  evidence  yet  obtained  concerning 
the  relations  of  particular  characters  and  particular 
chromosomes,  for  in  this  case  an  abnormal  distribu- 
tion of  the  sex  chromosomes  goes  hand  in  hand  with 
an  identical  abnormal  distribution  of  all  sex  linked 
factors.  It  was  found  that  females  from  a  certain 
strain  of  white-eyed  flies  gave,  on  out-crossing,  about 
5  per  cent,  of  unexpected  classes.  For  instance, 
one  of  the  white  females  crossed  to  a  red-eyed  male 
(wild  type)  produced  not  only  red-eyed  daughters 
and  white-eyed  sons,  as  expected,  but  also  a  few 
white-eyed  daughters  and  a  corresponding  number 
of  red-eyed  sons.  The  approximate  percentage  in 
which  these  classes  appeared  is  as  follows: 

Red   9  White  d"        White    9          Red  <f 

47.5%  47.5%          2.5%  2.5% 

In  general,  therefore,  there  were  95  per  cent,  of 
expected  forms  and  5  per  cent,  of  offspring  that 
were  apparently  inconsistent  with  expectation  on  the 
chromosome  theory.  Closer  inspection  of  these 


150  DISTRIBUTION    OF    THE    CHROMOSOMES 

results  showed  that  the  exceptions  could  be  explained, 
if,  occasionally,  the  two  X  chromosomes  failed  to 
disjoin  in  the  reduction  division,  both  passing  out  of 
some  of  the  eggs  of  the  white-eyed  mother  into  the 
polar  body,  or,  conversely,  both  remaining  in  the  egg. 
If  the  two  white-bearing  X's  should  remain  in  the  egg 
then  such  an  egg  fertilized  by  a  Y  sperm  would  give 
rise  to  a  white-eyed  daughter.  Likewise  the  no-X 
egg  fertilized  by  the  X  sperm  of  a  red-eyed  male 
would  give  a  red-eyed  son.  The  white  daughters 
would,  as  just  shown,  contain  two  X's  and  one  Y 
chromosome,  unlike  ordinary  daughters,  which  con- 
tain two  X's  only.  Since  in  these  females  there  are 
three  sex  chromosomes  instead  of  a  pair,  at  the 
reduction  division  two  must  pass  into  one  cell  and 
one  into  the  other.  This  division  might  take  place 

XY    X      Y  XX 

in  four  ways:     -^-.>  ^y'  XX  an(*  ~yr~  (representing 

the  egg  below  and  the  polar  body  above  in  each 
case).  The  first  two  types  of  reduction,  depending 
on  a  more  symmetrical  pairing  of  the  chromosomes, 
might  be  more  frequent  than  the  other  two  types. 
There  would  then  be  four  types  of  eggs  —  a  large 
number  of  X  and  XY  eggs,  and  a  few  XX  and  Y  eggs. 
Let  us  suppose  that  an  XXY  white  female  is  mated  to  a 
red  male.  The  progeny  produced  by  the  X  bearing 
sperm  would  be  : 

(I)  (2)  (3)  (4) 


9  red  <p  red  missing  d  red 


DISTRIBUTION    OF    THE    CHROMOSOMES         151 

The  same  series  of  eggs  fertilized  by  the  male- 
producing  sperm,  which  carries  a  Y  chromosome, 
would  give: 

(5)         .  (6)  (7)  (8) 


W 
I  I 


Cfwhite  cf  white  O  white  dies 

If  we  consider  these  eight  kinds  of  progeny  we 
see  that  the  exceptional  white  females  (7)  would  be 
expected  to  repeat  the  process  and  be  non-disjunc- 
tional.  This  is  what  actually  occurs,  for  all  white 
females  that  are  the  product  of  such  a  cross  do,  in 
fact,  give  non-disjunction  in  the  next  generation. 

The  red  males  (4)  are  an  exceptional  class  but 
should  not  give  exceptional  results  when  bred  to  any 
normal  female,  nor  should  they  transmit  non-dis- 
junction. This  has  been  shown  to  be  true. 

The  red  females  are  not  alike  in  composition,  half 
of  them  (1)  should  behave  like  normal  females 
heterozygous  for  white  and  the  other  half  (2)  should 
give  exceptions.  There  are  in  fact  found  to  be 
these  two  kinds  of  red  females  in  equal  numbers. 

The  white  males  (5)  and  (6)  are  not  alike;  one 
kind  (5)  is  normal  and  the  other  (6)  has  two  Y 
chromosomes.  The  latter  should  be  expected  to 
produce  some  XY  sperm.  These  sperm  would  give 
daughters  which  would  not  be  exceptions,  but  such 
females,  with  a  formula  XXY,  should  produce 
exceptions.  In  fact  from  half  of  the  white  males  (5 
and  6),  daughters  are  produced  that  give  non-dis- 
junction. 


152  DISTRIBUTION    OF   THE    CHROMOSOMES 

The  results  bear  out  to  a  remarkable  degree  the 
hypothesis  that  they  are  due  to  a  non-disjunction 
of  the  sex  chromosomes  caused  by  the  presence  of  a 
Y  chromosome  in  the  females. 

The  hypothesis  is  capable  of  verification  and 
Bridges  has  made  a  study  of  the  chromosomes  of  the 
non-disjunctional  females.  He  finds  that  such 


FIG.  49.  —  Group  of  chromosomes  of  an  XXY  female  of  a  non-disjunc- 

"" 


tional  "line. 

females  contain  an  extra  chromosome  whose  size 
and  position  show  that  it  is  a  supernumerary  sex 
chromosome.  The  normal  group  of  chromosomes  of 
the  female  of  Drosophila  ampelophila  is  shown  in 
Fig.  2,  and  a  group  from  a  non-disjunction  female 
in  Fig.  49.  They  differ  by  one  chromosome, 
namely,  the  extra  Y. 

One  additional  fact  must  be  mentioned.     If  an 
XXY  female  should  be  fertilized  by  an  XYY  male 


DISTRIBUTION    OF    THE    CHROMOSOMES  153 

some  females  would  be  produced  that  are  XX  YY, 
owing  to  the  union  of  an  XY  egg  with  an  XY  sperm 
or  an  XX  egg  with  a  YY  sperm.     One  such  female, 
was  found — she  had  two  X  and  two  Y  chromosomes. 

Here  then  is  a  case  that  seemed  at  first  to  be  in 
direct  contradiction  to  the  scheme  of  sex  linked 
inheritance  based  on  the  chromosome  hypothesis, 
which  proved,  however,  on  further  examination  to 
give  a  brilliant  confirmation  of  that  theory;  for  not 
only  can  the  hereditary  results  be  accounted  for,  but 
the  theory  on  which  they  were  based  was  directly 
confirmed  by  a  microscopical  study  of  the  chromo- 
somes themselves. 

Cases  indicating  non-disjunction  have  also  been 
obtained  in  Abraxas,  by  Doncaster.  As  stated  in 
the  chapter  on  Sex  Inheritance,  he  has  found  a  strain 
in  which  the  males  have  56  chromosomes — the 
normal  number,  but  the  females  have  only  55  instead 
of  56  chromosomes.  It  seems  reasonable,  then,  to 
suppose  that  such  females  arose  by  the  passing  of  the 
two  sex  chromosomes,  ZZ,  to  one  pole  (spermatocyte) 
leaving  none  at  the  other  pole  of  the  cell.  The  sperm 
resulting  from  the  no-Z  cell  fertilizing  a  Z  egg  would 
give  a  ZO  individual  which  would  be  a  female  with 
55  chromosomes.  All  the  daughters  of  the  ZO 
female  would  be  ZO  and  her  sons  ZZ  individuals: 
and  the  race  would  continue  in  this  fashion.  On 
the  other  hand,  if  the  ZZ  sperm  produced  by  non- 
disjunction  fertilized  a  W  egg,  a  male  WZZ,  corre- 
sponding to  the  XX Y  female  of  Drosophila,  would  be 
formed.  Such  a  male  would  give  rise  to  some  sperm 


154  DISTRIBUTION    OF    THE    CHROMOSOMES" 

carrying  both  Z  and  W,  and  if  such  a  ZW  sperm 
fertilized  a  zero  egg  of  the  55  chromosome  female,  a 
56  chromosome  female  would  be  produced.  Don- 
caster  actually  found  such  a  female  among  offspring 
from  a  cross  of  a  female  from  the  55  chromosome 
race  with  wild  type  male,  and  he  found  also  the 
genetic  exceptions  required  on  the  assumption  that 
this  male  was  a  WZZ  form. 


CHAPTER  VII 
MULTIPLE  ALLELOMORPHS 

The  meaning  of  the  term  multiple  allelomorphs 
may  be  illustrated  by  the  following  example : 

1.  If  a  white-eyed  male  of  Drosophila  is  mated  to 
a  red-eyed  female,  the  F2  ratio  of  3  reds  to  1  white  is 
explained  by  Mendel's  law,  on  the  basis  that  the 
factor  for  red  is  the  allelomorph  of  the  factor  for 
white. 

2.  If  an  eosin-eyed  male  is  mated  to  a  red-eyed 
female,  the  F2  ratio  of  3  reds  to  1  eosin  is  also  ex- 
plained if  eosin  and  red  are  allelomorphs. 

3.  If  the  same  white-eyed  male  is  bred  to  an  eosin- 
eyed  female,  the  F2  ratio  of  3  eosins  to  1  white  is 
again  explained  by  making  eosin  and  white  allelo- 
morphs. 

There  are  here  three  factors,  any  two  of  which 
may  meet,  and  whenever  they  do,  they  behave  as 
allelomorphs.  They  form  a  system  of  triple  allelo- 
morphs. 

On  the  chromosome  hypothesis  the  explanation  of 
this  relation  is  apparent.  A  mutant  factor  is  located 
at  a  definite  point  in  a  particular  chromosome;  its 
normal  allelomorph  is  supposed  to  occupy  a  corre- 
sponding position  (locus)  in  the  homologous  chromo- 
some. If  another  mutation  occurs  at  the  same  place, 

155 


156  MULTIPLE    ALLELOMORPHS 

the  new  factor  must  act  as  an  allelomorph  to  the  first 
mutant;  as  well  as  to  the  "parent"  normal  allelo- 
morph. 

Since  these  factors  have  the  same  location  they 
must  all  give  the  same  linkage  values  with  other  fac- 
tors. This  has  been  shown  to  be  true.  For  instance, 
the  factor  for  white  eye  color  of  Drosophila  is  very 
closely  linked  to  that  for  yellow  body  color.  The 
" distance"  between  them  is  1  unit,  which  means 
that  crossing  over  takes  place  about  once  in  a 
hundred  times.  Eosin  eye  color  gives  the  same 
crossing  over  frequency  with  yellow. 

White  eye  color  gives  with  miniature  wings  about 
33  per  cent,  crossing  over.  Eosin  gives  the  same 
value  with  miniature. 

White  gives  44  per  cent,  of  crossing  over  with 
bar  eye.  Eosin  has  the  same  value.  Similar  rela- 
tions hold  for  all  of  the  characters  of  the  first  group; 
they  all  have  the  same  linkage  values  for  eosin  that 
they  have  for  white.  This  example  indicates  that 
the  conception  of  allelomorphs  should  not  be  limited 
to  two  different  factors  that  occupy  identical  loci 
in  homologous  chromosomes,  but  that  there  may  be 
three,  as  above,  or  even  more  different  factors  that 
stand  in  such  a  relation  to  each  other.  Since  they 
lie  in  identical  loci  they  are  mutually  exclusive,  and 
therefore  no  more  than  two  can  occur  in  the  same 
animal  at  the  same  time.  This  is  both  demonstrated 
by  the  facts  and  postulated  by  the  chromosomal 
mechanism. 

On  a  priori  grounds  also  it  is  reasonable  to  suppose 


MULTIPLE    ALLELOMORPHS  157 

that  a  factor  could  change  in  more  than  one  way, 
and  thus  give  rise  to  multiple  allelomorphs,  unless 
it  is  supposed  that  the  only  change  possible  in  a  factor 
is  a  complete  loss  of  the  factor,  as  postulated  in  the 
presence  and  absence  theory. 

There  is,  however,  an  alternative  theory  to  that  of 
multiple  allelomorphism.  This  alternative  is  com- 
plete linkage.  The  numerical  result  can  be  equally 
well  explained  if,  instead  of  occupying  identical  loci, 
the  factors  are  so  near  together  that  they  never 
(or  very  rarely)  cross  over.  For  reasons  that  will  be 
given  later  we  are  inclined  to  think  that  the 
explanation  of  multiple  allelomorphism  is  in  most 
cases  the  more  probable  one,  but  the  arguments  in 
favor  of  this  view  may  be  deferred  until  the  facts 
have  been  described. 

There  is  a  general  relation  that  so  far  holds  for 
all  cases  in  which  multiple  allelomorphs  have  been 
discovered,  namely,  that  the  factor-differences  pro- 
duce similar  effects.  All  of  the  following  examples 
illustrate  this  relation. 

In  rabbits  (Fig.  50)  the  Himalayan  pattern  has 
been  shown  to  behave  as  a  recessive  to  self-color  and 
a  dominant  to  albino.  Any  two  of  these  three  types 
of  pigment  formation  and  distribution  give  a  3:1 
ratio  in  F2  but  no  two  of  them,  when  crossed,  ever 
produce  the  third  genetic  type.  In  other  words  the 
factors  behave  as  though  allelomorphic,  for  only  two 
can  be  gotten  into  any  one  individual.  A  similar  re- 
lation has  been  described  by  Baur  in  the  columbine, 
where  three  types  of  leaves,  green,  variegated  (green 


158 


MULTIPLE    ALLELOMORPHS 


FIG.  50. — Himalayan,  black  and  white  rabbits.     The  factor  that  stands 
for  each  is  allelomorphic  to  the  others. 


MULTIPLE    ALLELOMORPHS  159 

and  yellow),  and  yellow  form  a  triple  system. 
Emerson's  case  for  pod  and  leaves  in  beans — green 
pods,  green  leaves;  yellow  pods,  yellow  leaves; 
yellow  pods,  green  leaves — also  fulfill  the  conditions 
of  a  triple  allelomorph  system.  Shull  has  reported  a 
case  in  Lychnis  which  he  interprets  as  due  to  triple 
allelomorphs  for  sex-determining  factors.  Two  of 
them  give  reversible  mutations  as  have  white  and 
eosin  in  Drosophila. 

Cases  in  which  more  than  three  allelomorphs  have 
been  found  may  next  be  considered.  The  cases  seem 
to  show  that  here  also  the  same  character  is  affected 
by  each  of  the  mutant  factors  that  form  the  multiple 
system.  In  a  few  instances  the  characters  have  been 
recognized  as  due  to  multiple  allelomorphs,  but  in 
most  of  them  no  sufficient  interpretation  has  been 
offered  or  else  the  explanation  of  complete  linkage 
has  been  advanced. 

Tanaka  has  reported  a  case  in  the  silkworm  moth 
which  seems  best  interpreted  as  one  of  quadruple 
allelomorphs.  The  four  larval  patterns  called  striped, 
moricaud,  normal,  and  plain  (Fig.  51),  are  the  char- 
acters involved.  Besides  showing  the  ordinary  be- 
havior of  multiple  allelomorphs  when  mated  together 
these  characters  show  linkage  to  another  pair  of 
factors  (for  yellow  and  white  cocoon  color).  So  far 
as  the  data  go,  the  strength  of  this  linkage  seems  to 
be  the  same  in  all  combinations  tested. 

In  mice  it  has  been  shown  (Cuenot,  Morgan, 
Sturtevant,  and  Little)  that  yellow,  black,  gray  with 
gray  belly  (wild  type),  and  gray  with  white  belly 


160 


MULTIPLE    ALLELOMORPHS 


(second  wild  type)  are  allelomorphs.  It  will  be  ob- 
served here  that  the  factor  in  the  wild  type  gray 
mouse  is  responsible  for  the  appearance  in  each 
hair  of  the  three  pigments,  chocolate,  yellow  and 


r  f 


" ' 


FIG.  51. — Four  allelomorphic  characters  in  the  silkworm:  a,  Chinese 
striped  yellow;  b,  Chinese  moricaud  yellow;  c,  Japanese  normal  yellow; 
d,  Chinese  plain  white. 

black.  Gray  is  therefore  a  mosaic  effect,  for  these 
colors  are  stratified  in  each  hair  from  the  base  out- 
ward in  the  order  above  named.  The  allelomorphic 
factor  for  yellow  gives  rise  to  only  one  of  these 


MULTIPLE    ALLELOMORPHS  161 

colors,  although  the  others  may  to  some  extent 
appear,  especially  in  old  mice.  The  third  allelo- 
morph produces  only  black  or  at  least  the  chocolate 
pigment,  if  present,  is  obscured  by  the  darker  color. 
Finally,  the  fourth  allelomorph  produces  gray  on 
the  back  and  sides  while  the  belly  is  pure  white 
(the  under  hair  is  black).  This  series  illustrates 
how  allelomorphs  of  the  same  locus  may  not  only 
determine  the  color,  but  also  act  to  determine  where 
a  color  is  to  develop.  The  allelomorphs  differ  there- 
fore in  regard  to  what  part  of  the  body  they  affect, 
or  the  time  in  ontogeny  when  they  act,  as  in  the 
banded  hair  of  the  gray  mouse. 

This  case  serves,  therefore,  as  an  excellent  intro- 
duction to  the  cases  that  Emerson  has  described  in 
corn  (maize),  in  which  the  red  color  of  the  grain 
(pericarp),  cob,  silk,  and  husk  furnish  a  wonderful 
series  of  character  combinations  that  can  be  ex- 
plained on  the  multiple  allelomorph  hypothesis. 
Emerson  adopted  the  hypothesis  of  complete  linkage, 
but  the  same  arguments  as  used  in  other  cases  lead 
us  to  prefer  the  alternative  of  multiple  allelomorphs. 
In  some  varieties  of  corn  the  grain,  the  cob,  the  silk, 
and  the  husk  are  all  red;  in  others,  all  white;  in  others 
the  grain  may  be  red,  the  cob,  silk,  and  husk  white;  in 
others,  the  grain  may  be  white  and  the  rest  red. 
Practically  all  possible  combinations  are  known,  and 
so  far  as  tested  the  combinations  that  go  in  through 
the  two  parents  come  out  in  F2  according  to  expecta- 
tion, i.e.,  they  give  no  new  gametic  recombinations. 
If  we  assume  that  there  is  a  system  of  allelomorphs, 


162  MULTIPLE    ALLELOMORPHS 

such  that  one  affects  one  combination  of  parts, 
another  a  different  combination,  the  results  find  a 
simple  and  consistent  explanation.  It  may  seem 
strange  at  first  that  a  factor  may  make  the  cob  red 
and  not  color  the  grain  or  husk,  while  another 
allelomorph  may  make  the  grain  and  husk  red  but 
not  affect  the  cob  color,  but  it  is  no  more  strange  than 
that  one  factor  determines  one  distribution  of  the 
pigment  over  the  coat  and  even  in  each  hair  of  the 
gray  mouse  and  another  one  determines  another 
distribution. 

Equally  striking  is  the  series  of  forms  of  the  grouse 
locust  (Paratettix)  that  Nabours  has  recently  studied. 
Nine  true  breeding  forms  that  are  found  in  nature 
were  studied.  They  differ  markedly  in  color  pattern 
(Fig.  52)  but  each  color  pattern  behaves  as  a  -unit  in 
heredity.  The  hybrid  is  in  a  sense  intermediate,  the 
color  characters  of  each  parent  being  superimposed. 
In  fact  Nabours  finds  that  simple  inspection  of  the 
hybrid  suffices  to  show  which  forms  were  its  parents. 
In  the  germ  cells  of  the  hybrid  the  two  parental  color 
types  segregate  as  units.  The  resulting  F2  types 
are  in  the  1  :2  : 1  ratio.  It  is  obvious,  since  only  two 
of  the  color  types  can  exist  in  the  same  individual, 
and  since  they  separate  in  the  germ  cells,  that  the 
condition  of  multiple  allelomorphism  is  fulfilled. 

All  Nabour's  crosses  relating  to  color  pattern  (with 
some  possible  exceptions)  follow^  the  plan  just  out- 
lined. The  case  at  first  sight  appears  unique  in  that 
the  color  pattern  of  each  type  is  complex  in  the  sense 
that  different  parts  of  the  body  are  differently  affected 


MULTIPLE    ALLELOMORPHS 


163 


and  in  that  in  most  cases  the  hybrid  shows  at  the 
same  time  the  characters  of  each  parent.  Both  of 
these  peculiarities  occur  in  other  cases,  however, 


B 


BC 

FIG.  52. — Four  types,  A,  B,  C,  I,  of  Paratettix.     Below   are   hybrids 
between  A  and  B,  B  and  C,  and  B  and  7.     (After  Nabours.) 

as  in  Emerson's  corn  for  instance,  although  nowhere 
perhaps  so  strikingly  as  in  Paratettix. 

In  any  attempt  to  decide  between  the  two  alter- 


164  MULTIPLE    ALLELOMORPHS 

native  views  of  identical  loci  and  of  complete  linkage 
the  method  of  origin  of  the  mutant  allelomorph  is  a 
matter  of  prime  importance.  Emerson  has  described 
one  type  (" variegated"  corn)  in  which  a  mutation 
(to  red)  occurs  frequently.  This  mutation  is  of 
such  a  sort,  as  Emerson  points  out,  that,  on  the 
theory  of  complete  linkage,  it  must  involve  the  muta- 
tion of  two  factors  at  the  same  time.  On  the  theory 
of  multiple  allelomorphs  only  one  mutation  is 
necessary  each  time  the  change  occurs.  Fortunately 
we  have  complete  information  concerning  the  origin 
of  the  types  of  Drosophila  that  fall  into  this  category. 
One  of  these  may  now  be  given  in  detail  before 
attempting  to  decide  between  the  claims  of  the  rival 
explanations. 

In  1911  a  few  males  with  white  eyes  arose  in  a 
culture  of  red  eyed  flies.  From  them  the  stock  of 
white  eyed  flies  was  obtained  by  the  usual  procedure. 
In  1912,  in  a  culture  of  white  eyed  flies  having  also 
miniature  wings  and  black  body  color,  a  male  ap- 
peared that  had  eosin  eyes.  He  also  had  miniature 
wings  and  black  body  color,  so  that  there  could  be  no 
question  of  his  origin  from  this  particular  stock. 
The  eosin  stock  is  descended  from  this  male. 

In  1913,  in  a  cross  between  vermilion  eyed  flies  and 
wild  flies  several  males  appeared  in  F2  whose  eyes 
were  quite  different  from  vermilion.  Analysis  of 
the  case  showed  that  a  mutation  had  taken  place  in 
the  stock  having  vermilion  eye  color.  The  new  color 
proved  to  be  a  double  recessive,  for  vermilion  and 
for  a  color  called  cherry.  The  new  mutation  had 


MULTIPLE    ALLELOMORPHS  165 

not  occurred  at  the  locus  of  the  vermilion  factor, 
however,  but  at  another  locus  where  there  had  been  a 
normal  factor.  Subsequent  work  with  the  cherry 
eye  color  showed  that  it  was  allelomorphic  to  white 
and  to  eosin,  the  three  eye  colors  and  their  normal 
allelomorph  forming  a  quadruple  system. 

To  the  preceding  history  must  be  added  cases  of 
the  return  mutation  from  eosin  to  white.  Such  a 
mutation  occurred  in  1914  in  a  culture  of  eosin  flies 
with  miniature  wings.  The  parents  had  been  treated 
with  alcohol,  but  there  is  no  evidence  to  show  that 
the  alcohol  had  any  connection  with  the  event.  A 
single  white  eyed  male  appeared  among  many 
hundred  eosin  brothers  and  sisters.  The  male  had 
miniature  wings.  When  crossed  by  ordinary  white 
it  produced  white  through  two  generations.  There 
can  be  little  doubt  that  it  is  the  same  white  as 
the  original  white.  In  a  pure  bred  stock,  eosin 
tan  vermilion,  a  few  males  were  found  which  had 
a  white  eye  color  instead  of  the  cream  color  of 
eosin  vermilion.  These  flies  mated  to  white  stock 
gave  white  offspring  for  two  generations.  Here  the 
case  was  checked  by  two  control  characters,  for 
the  new  white-eyed  males  showed  tan  body  color 
and  were  proved  to  carry  vermilion.  In  these 
controlled  cases  the  mutation  took  place  in  the 
reverse  direction  from  the  original  one.  Three 
other  cases  of  eosin  returning  to  white  which  are 
apparently  not  explainable  by  contamination  are  also 
recorded. 

The  appearance  of  eosin  in  the  white-eyed  stock 


166  MULTIPLE    ALLELOMORPHS 

might  be  interpreted  to  mean  that  a  mutation  in  eye 
color  had  appeared  in  the  white-eyed  stock  in  a 
factor  located  near  the  factor  for  white  ("completely 
linked"  with  it)  and  that  the  effect  of  this  new  factor, 
combined  with  that  of  the  factor  for  white,  which 
was  already  there,  gave  the  color  that  we  call  eosin. 
Eosin  from  this  point  of  view  would  be  due  to  two 
consecutive  mutations  of  completely  linked,  neigh- 
boring loci.  This  interpretation  of  two  consecutive 
mutations  can  not  be  made  in  the  case  of  cherry, 
however,  for  cherry  arose  from  red  by  one  step,  just 
as  did  white;  yet  cherry,  like  eosin,  when  mated  to 
white,  does  not  give  rise  to  offspring  that  are  red. 
It  would  follow  on  the  complete  linkage  view  that 
cherry  and  white  differ  from  red  by  the  same  factor, 
but  since  they  are  not  alike,  that  one  of  them  must  differ 
from  red  by  still  another  factor.  Since  each  arose 
from  red  immediately,  it  would  follow  that  one  of 
them  must  have  arisen  by  a  simultaneous  mutation 
in  two  factors  completely  linked  and  affecting  the 
same  character.  All  these  assumptions  must  be 
made  on  the  theory  of  complete  linkage,  but  are 
avoided  on  the  alternative  theory  of  multiple 
allelomorphs. 

Exactly  the  same  argument  applies  in  the  case  of 
two  other  triple  allelomorph  systems  of  Drosophila. 
The  recessive  mutants  pink  and  peach  colored  eyes 
each  arose  independently  from  red  eyed  flies,  yet 
when  crossed  do  not  give  red,  but  a  color  intermediate 
between  pink  and  peach.  Secondly,  sooty  body 
color  arose  in  wild  stock,  although  it  was  found  only 


MULTIPLE    ALLELOMORPHS 


167 


after  the  stock  had  been  crossed  to  ebony,  with  which 
it  is  allelomorphic.  Here  too  the  mutant  forms 
though  both  recessive  to  normal  do  not  give  normal 
gray  color  when  crossed  together,  but  a  color  inter- 
mediate between  sooty  and  ebony.  In  both  of  these 
cases  the  complete  linkage  view  would  require  that 
one  of  the  mutant  types  had  originated  by  a  muta- 
tion in  two  factors  at  once.  There  is  still  another  set 


FIG.  53. — The  abdomen  of  normal  a,a  ,  and  spot,  b,b',  males.     The  other 
allelomorph  is  yellow  (not  shown  here). 

of  triple  allelomorphs  known  in  Drosophila,  namely, 
yellow  and  spot  (Fig.  53)  and  their  normal  allelo- 
morph. The  above  argument  does  not  apply  to 
this  case,  however,  for  although  spot  and  yellow  are 
both  recessive  to  gray  and  give  yellow  when  crossed 
to  each  other,  spot  originated  in  flies  containing 
already  the  allelomorph  for  yellow. 

The  reasons  may  now  be  given  that  incline  us  to 
think  that  the  theory  of  identical  loci  is  much  more 


168  MULTIPLE    ALLELOMORPHS 

probable  for  the  cases  known  than  is  that  of  complete 
linkage  (in  the  sense  defined) .  No  one  of  the  reasons 
is  in  itself  conclusive,  but  taken  together  they  weight 
the  scales  heavily  on  one  side. 

1.  When  two  mutants  that  depend  on  "  multiple 
allelomorphs"  are  crossed  they  give  in  FI  a  type  that 
is  like  one  or  the  other  of  the  two  mutants,  or  an 
intermediate  type.     This  type  is  scarcely  ever  like  the 
original  (or  wild)  type.     In  this  respect  they  differ 
from    other    recessive    mutant    types    which    when 
crossed  together  give  the  wild  type.     We  understand 
why  in  the  latter  cases  the  wild  form  is  recovered. 
It  is  because  each  mutant  type  contains  besides  its 
mutant  factor  the  normal   (dominant)   allelomorph 
of  the  other  type.     Hence  the  original  type  is  re- 
constituted in  the  cross,  as  has  been  already  stated. 
But  when  two  mutant  allelomorphs  occupying  the 
same  locus  are  brought  together  neither  of  them 
brings    in    the    normal    allelomorph    of    the    other; 
hence  the  wild  type  is  not  reconstituted.     If  the 
cases    in   which    these    allelomorphic    factors    arose 
independently  are  not  cases  of  identical  loci  then  the 
explanation   involves  the  occurrence  of  two  muta- 
tions at  the  same  time,  as  explained  in  the  case  of 
cherry. 

2.  It  is  a  characteristic  of  " multiple  allelomorphs" 
that  the  same  character  is  affected.     Nearness  of 
factors  in  the  chromosome  will  not  explain  this  fact 
unless  nearness  means  the  same  factorial  basis,  for 
in  the  other  mutants  that  we  have  obtained,  nearness 
of  factors  is  in  no  way  related  to  the  kind  of  character 


MULTIPLE    ALLELOMORPHS  169 

or  part  of  the  body  that  is  affected.  It  seems  there- 
fore more  probable  that  this  peculiar  fact  connected 
with  multiple  allelomorphs  means  that  the  same 
portion  of  the  chromosome  is  changed  in  one  or 
another  direction. 

3.  It  is  true  that  a  very  wide  range  of  linkage  values 
has  been   obtained,  that  extends  from  almost  free 
segregation  to  less  than   1  per  cent,  of   crossovers. 
However,  if  we  should  construct  a  curve  showing  the 
number    of    cases    exhibiting    the    various    possible 
linkage  values,  the  number  showing  complete  linkage 
or,  as  we  should  say,  multiple  allelomorphism,  would 
be  far  in  excess  of  the  number  of  these  to  be  expected 
from  the  general  shape  of  the  rest  of  the  curve.     This 
indicates  that  multiple  allelomorphs  are  in  a  class 
by  themselves,  not  merely  extreme  cases  of  the  same 
type  as  an  ordinary  linkage  case. 

4.  There  is  an  a  priori  consideration  that  may  not 
be  out  of  place  in  the  argument.     There  is  no  suffi- 
cient reason  for  supposing  that  only  one  sort  of 
mutation  can  occur  in  a  given  locus  in  the  chromo- 
some.    If  the  basis  of  the  chromosome  is  a  chain  of 
chemically  complex  substances  (e.g.,  proteins),  any 
slight  addition  or  loss  or  even  re-arrangement  of  the 
atoms  in  the  molecules  of  a  bead  in  such  a  chain 
might  well  produce  an  effect  on  the  organism,  and 
perhaps  a  more  marked  effect  on  that  particular 
character   that   stands   in   closest   relation   to    that 
chemical  body.     Since  we  know  that  mutations  and 
even  "reverse"  mutations  actually  occur,  it  would  be 
indeed   strange   if   only   one   kind   of   change   were 


170  MULTIPLE    ALLELOMORPHS 

possible  in  a  given  locus.  But  if  more  than  one 
kind  of  change  did  take  place  in  a  locus,  a  series  of 
multiple  allelomorphs  would  result. 

The  ability  of  the  theory  of  multiple  allelomorphs 
(identical  loci)  to  explain  the  peculiarities  of  so 
many  cases  in  such  widely  separated  fields  proves  the 
usefulness  of  the  hypothesis.  Although  the  theory 
of  complete  linkage  also  will  cover  the  numerical 
results  in  these  cases  (and  some  of  the  simpler  cases 
cited  may  prove  to  fall  under  this  head)  there  is  the 
very  strong  first-hand  evidence  that  has  just  been 
given  that  makes  the  theory  of  multiple  allelomorphs 
more  probable  than  the  former  theory.  It  is  im- 
portant to  recognize  that  there  is  this  strong  evidence 
in  favor  of  multiple  allelomorphs,  quite  aside  from 
special  cases  of  complete  linkage,  for,  as  will  be  shown 
in  the  next  chapter,  there  are  some  far-reaching 
consequences  of  the  theory  of  multiple  allelomorphs. 

A  word  may  not  be  out  of  place  here  concerning 
the  relation  of  the  theory  of  multiple  allelomorphs  to 
the  question  of  the  variability  of  factors.  The  fact 
that  more  than  one  change  may  take  place  in  the 
material  at  a  given  locus  must  not  be  taken  to 
mean  that  the  material  is  undergoing  continuous 
fluctuating  variability,  for  such  mutations  occur 
rarely  and  the  factors  later  behave  as  do  others. 
In  fact  in  only  one  case  (i.e.,  Emerson's  variegated 
corn)  do  mutations  appear  frequently  at  a  given 
locus.  But  even  in  such  case  the  change  can  not 
properly  be  said  to  be  fluctuating,  but  is  of  a  fixed 
nature,  and  when  it  has  once  occurred  the  new  factor 


MULTIPLE    ALLELOMORPHS  171 

is  no  more  subject  to  mutation  than  are  other  factors, 
i.e.,  the  factor  has  lost  its  unusual  instability. 

There  is  no  a  priori  answer  possible  to  the  question 
as  to  whether  a  mutation  having  occurred,  a  further 
mutation  of  the  mutated  factor  is  more  likely  to 
occur,  for  it  is  conceivable  that  while  in  one  case 
the  new  factor  might  be  unstable,  in  another  case  it 
might  be  even  more  stable  than  the  original  one. 
In  regard  to  the  other  question,  as  to  whether  a  par- 
ticular locus  is  more  liable  to  mutate,  the  work  on 
Drosophila  shows  that  certain  loci  do  mutate  more 
often  than  do  others,  and  this  is  shown  not  only  in 
the  recurrence  of  the  same  mutation,  but  also  in  the 
occurrence  of  multiple  allelomorphs. 


CHAPTER  VIII 
MULTIPLE  FACTORS 

The  term  " multiple  factors"  has  come,  in  prac- 
tice, to  be  applied  usually  to  cases  in  which  two  or 
more  factor-differences  occur,  all  of  which  produce 
similar  effects.  The  frequency  with  which  such 
cases  are  found  is  not  surprising,  since,  on  the 
factorial  interpretation  of  heredity,  it  is  apparent 
that  many  factors  must  contribute  toward  the 
making  of  every  character.  For  example,  the  char- 
acter, eye  color,  can  appear  only  after  the  complex 
series  of  developmental  reactions  has  taken  place, 
whereby  in  turn  head,  eyes,  pigment  cells,  etc.,  have 
been  formed,  and  so  this  character  must  ultimately 
depend  on  all  the  factors  affecting  these  processes. 
There  must,  besides,  be  many  factors  that  operate 
in  a  more  direct  manner  in  the  production  of  nearly 
every  character,  since  on  analysis  even  the  simplest 
character  usually  proves  to  be  the  resultant  of  many 
components,  both  physical  and  chemical.  Thus 
the  color  of  the  eye  must  depend,  among  other 
things,  on  the  size  of  the  pigment  granules,  on  their 
number  and  on  their  color,  and  the  color  of  the 
pigment  may  in  turn  be  dependent  on  reactions  in 
which  many  substances  take  part.  It  is  therefore 
evident  that  an  apparently  simple  character,  like  eye 

172 


MULTIPLE    FACTORS  173 

color,  involving  only  one  organ,  is,  so  far  as  its  mode 
of  inheritance  is  concerned,  in  no  wise  different  in 
kind  from  a  complex  character  like  stature  which,  as 
Bateson  pointed  out  in  1902,  must  depend  on  all 
factors  affecting  length  of  head,  neck,  trunk,  or  legs. 

In  the  case  of  eye  color  in  Drosophila,  more  than 
25  factor-differences  have  arisen  by  mutation.  Most 
of  these  factor-differences  are  dissimilar  in  their 
effects  upon  the  eye  color — thus,  one  differentiates 
a  purple  eyed  fly  from  the  red,  another  differentiates 
vermilion  from  red,  another  white  from  red,  and  so 
on.  It  so  happens,  however,  that  two  mutations 
occurred,  one  in  the  sex-linked  group,  and  one  in  the 
third,  each  of  which  changed  the  red  eye  to  a  pink 
color.  It  is  to  such  cases  only — where  factor- 
differences  produce  the  same  or  very  similar  effects, 
or  effects  that  differ  only  in  degree — that  the  term 
" multiple  factors"  has  come  to  be  specifically 
applied.  It  should  be  recognized  that  this  restric- 
tion of  the  term  is  arbitrary,  but  there  is  a  practical 
advantage  in  grouping  these  particular  cases  to- 
gether under  a  common  heading,  because  crosses 
involving  several  factor-differences  that  are  similar 
in  effect  give  peculiar  ratios  and  present  certain 
difficulties  to  a  factorial  analysis,  not  commonly  met 
with  elsewhere. 

In  the  above  illustration  of  the  sex-linked  and 
third  chromosome  pinks  the  two  factor-differences 
were  not  present  in  the  same  cross,  and  their  in- 
heritance was  worked  out  separately.  They  were 
shown  to  be  different  factors,  not  by  their  behavior 


174  MULTIPLE    FACTORS 

with  reference  to  each  other,  but  by  their  different 
linkage  values  with  other  factors. 

An  example  of  a  cross,  involving  at  the  same  time 
two  factor-differences  which  have  similar  effects,  is 
Nilsson-Ehle's  cross  of  dark  brown  oats  having  two 
dominant  factors  for  dark  glumes  with  white-glumed 
plants  having  the  two  recessive  allelomorphic  factors 
for  light  color.  The  expected  F?  ratio  is  9  double 
dominant  dark  browns  (AB) :  3  light  browns  having 
the  first  recessive  and  the  second  dominant  (aB) :  3 
light  browns  having  the  first  dominant  and  the 
second  recessive  (Ab) :  1  double  recessive  white  (ab) . 
Since  the  two  factor-differences  produce  similar 
results,  however,  the  light  browns,  aB  and  Ab,  are 
indistinguishable;  counting  these  two  classes  to- 
gether, a  9 : 6 : 1  ratio  results.  The  9  double  dominants 
were  distinguishable  from  the  6  single  dominants, 
the  pigment  being  dark  brown  in  the  9  cases  where 
both  factors  for  dark  glumes  were  present  and  both 
factors  for  light  glumes  absent,  but  only  light  brown 
in  the  6  cases  where  one  light  and  one  dark  factor 
were  present.  Similarly  the  1  double  recessive, 
having  both  light  and  no  dark  factors,  was  much 
lighter  even  than  the  6  light  browns.  This  result 
may  be  described  by  saying  that  the  effects  of  the 
factors  for  dark  and  for  light  were  all  cumulative 
or  summative,  two  darks  producing  a  blacker  pig- 
ment than  one,  and  two  lights  a  paler  color  than  one. 

In  many  cases,  multiple  factors  do  not  give  results 
that  may,  in  the  above  sense,  be  called  cumulative. 
For  example,  if  a  white-flowered  sweet  pea  (ab) 


MULTIPLE   FACTORS  175 

having  two  pairs  of  recessive  factors  for  white  is 
crossed  with  a  colored  sweet  pea  (AB),  it  is  found 
when  the  9AB:  3aB:  3Ab:  lab  individuals  appear  in 
F2  that  the  aB  and  Ab  plants,  having  only  one 
factor  for  white  and  one  for  red,  are  just  as  white  as 
the  ab  plants.  In  other  words,  the  ab  class  can  show 
no  cumulative  effect  of  the  two  white  factors.  Since 
the  three  latter  classes  all  look  white,  they  are  added 
together  in  the  count,  and  a  ratio  of  9  reds :  7  whites 
results. 

It  is  commonly  said  that  this  result  is  due  to  the 
occurrence  of  two  factors  "for  red"  (the  dominants, 
A  and  B),  neither  of  which  alone  is  sufficient  to 
produce  any  effect  (since  Ab  and  aB  look  no  different 
from  ab),  but  which,  when  present  together,  act  as 
complements  to  each  other  and  thus  produce  the  red 
color.  Such  an  interpretation  fails,  however,  to  take 
into  consideration  the  possible  effects  of  the  recessive 
factors  "for  white"  (a  and  b).  It  is  therefore  un- 
warranted, unless  the  "presence  and  absence"  view 
be  accepted,  namely,  that  the  dominants  are  the 
only  real  factors,  the  recessives  being  mere  absences. 
It  would  likewise  be  unwarranted,  of  course,  to 
ascribe  the  results  purely  to  the  recessive  factors,  and 
so  to  conclude  the  similarity  of  aB  and  Ab  to  ab  was 
due  to  the  fact  that  a  and  b  were  non-cumulative  in 
their  effects.  Neither  of  these  methods  of  describing 
the  case  should  therefore  be  regarded  as  more  than  a 
shorthand  statement  of  the  empirical  facts. 

In  the  cross  of  Bursa  which  follows,  Shull,  using 
the  presence  and  absence  scheme,  treated  the  case 


176 


MULTIPLE    FACTORS 


as  one  of  two  similar  dominant  factors  producing  a 
non-cumulative  result.  (Here,  then,  the  9AB  re- 
semble the  3aB  and  3Ab  individuals  and  a  15:1  ratio 
results.)  To  those  who  reject  the  idea  that  domi- 
nance implies  presence,  recessiveness  absence,  there 
is  no  great  distinction  between  this  case  and  that  of 


CD- 


CJ- 


of  —  CD 


CJ 

1 


CD.  CD 


f 


:D  .  CD 


CJ  .  cD 


FIG.  54. — Diagram  showing  the  kinds  and  composition  of  the  F2  capsules 
of  Bursa  bursa-pastoris.     (After  Shull.) 

the  two  whites  with  a  9 : 7  ratio.  Shull  found  that 
when  a  plant  of  Bursa  bursa-pastoris  with  round 
capsules  is  crossed  to  one  with  triangular  capsules, 
the  round  is  recessive  to  triangular  in  FI.  In  F2  the 
round  reappears  only  once  in  sixteen  times  (Fig.  54). 
Thus  in  this  cross  round  may  be  treated  as  the  result- 
ant of  the  two  recessive  factors,  either  of  which  by 


MULTIPLE    FACTORS  177 

itself  does  not  change  the  triangular  type,  as  shown  by 
the  fact  that  both  single  recessives  are  triangular  in 
type  and  are  identical  in  appearance  with  the  double 
dominant.  Only  where  the  two  recessives  occur  in 
the  same  individual  does  the  type  change  to  round. 

Six  families  were  bred  from  the  FI,  and  gave  the 
following  counts: 

Triangular  Round  Ratio 

507  30  16.9:1 

146  4  36.5:1 

48  3  16.1:1 

179  9  19.9:1 

1743  72  24.2:1 

159  7  2? . 7 : 1 


Totals  2782  125  22.3:1 

Expected        2725  182  15.0:1 

The  actual  ratios  range  from  16  :1  to  36.5  :1,  which 
exceed  the  expected  ratio  of  15  : 1.  Nevertheless,  the 
deficiency  in  the  round  class  is  probably  due  to  the 
lower  viability  of  the  round-capsuled  type,  for  in 
later  cultures  where  the  conditions  were  more 
favorable  the  expected  15:1  ratios  are  more  nearly 
realized.  That  15 : 1  is  the  true  ratio  is  shown  by 
tests  that  were  applied  to  these  F2  plants.  In  Fig. 
54,  the  16  classes  (15  :1)  of  F2  individuals  are  repre- 
sented. Within  each  square  is  also  given  the  genetic 
composition  of  the  class.  The  letter  "c"  stands  for 
one  of  the  recessive  factors,  and  the  letter  "d"  for 
the  other  factor.  Both  of  these  recessive  factors 
acting  in  conjunction  produce  the  round  capsules  ccdd. 
Beneath  each  figure  is  given  the  expected  ratio  for 


178  MULTIPLE    FACTORS 

the  next  generation  when  the  plant  of  that  composi- 
tion is  self -fertilized.  It  will  be  observed  that  the 

1 : 0  ratio  is  expected  7  times. 

3  : 1  ratio  is  expected  4  times. 

15  : 1  ratio  is  expected  4  times. 

0  : 1  ratio  is  expected  1  time. 

This  test  was  applied  by  Shull  to  his  F2  plants  of  the 
triangular  type.  There  were  seven  families  that 
gave  a  1 : 0  ratio,  four  that  gave  approximately  a  3  : 1 
ratio,  and  six  that  gave  a  15  : 1  ratio.  These  results 
are  in  fair  accord  with  the  expected  numbers  given 
above. 

When  a  further  test  was  carried  out  by  breeding 
from  the  six  15:1  families  of  the  F3  group  above 
(which  should  be  expected  to  give  the  same  results 
as  the  F2  class,  because  they  have  the  same  composi- 
tion), the  ratios  obtained  were  as  follows: 

1 :0  ratio  expected  35;  realized  39. 

3  : 1  ratio  expected  20;  realized  12. 

15  :1  ratio  expected  20;  realized  26. 

The  results  agree  again  fairly  well  with  the  expecta- 
tion. 

A  second  test  is  found  in  self -fertilizing  plants  from 
families  that  gave  a  3 : 1  ratio.  As  the  diagram  shows 
these  contain  only  the  one  ("c")  or  the  other  ("d") 
factor,  they  should  give  only  homogeneous  families 
and  3  : 1  families — never  15 : 1  families.  This  result 
also  was  obtained. 

Nilsson-Ehle  found  that  three  recessive  factors 
must  combine  to  produce  an  effect  which,  in  the 


MULTIPLE    FACTORS  179 

following  case,  is  the  production  of  a  white-seeded 
wheat.  A  cross  between  white-seeded  and  red- 
seeded  wheat  gave  in  F2  one  white  to  sixty-three  reds, 
showing  that  three  independent  recessive  factors 
were  involved. 

Nilsson-Ehle  also  found  that  in  oats  a  type  without 
ligules  reappeared  in  F2  in  such  a  ratio  that  four 
recessive  factors  must  have  combined  to  have  pro- 
duced the  type  without  ligules.  East  found  certain 
kinds  of  yellow  corn  that  gave  in  F2  fifteen  yellows 
to  one  white.  We  may  here  also  interpret  the  white 
as  the  double  recessive.  East  has  pointed  out  that 
in  crosses  of  certain  strains  of  red  corn  white  appears 
in  F2  in  such  a  way  as  to  suggest  that  three  or  possi- 
bly four  recessive  factors  combine  to  produce  white. 

In  other  cases  of  multiple  factors,  the  two  factor- 
differences  differ  in  the  intensity  of  their  effect,  and 
so  in  F2  the  two  classes  aB  and  Ab  can  be  distin- 
guished from  each  other,  and  a  9:3:3:1  ratio  there- 
fore results.  In  some  of  these  cases,  however,  the 
factors  are  in  a  sense  non-cumulative  in  that  one  of 
the  factor-differences  produces  no  effect  when  a  given 
allelomorph  of  the  other  pair  of  factors  is  present. 
Thus,  in  the  ratio  9AB:3aB:3Ab:lab  if,  in  the 
presence  of  b,  a  and  A  produce  no  different  effect 
there  would  be  a  ratio  of  9:3:4.  This  is  true  in  a 
cross  of  a  black  mouse  (AB)  with  a  white  mouse 
carrying  both  the  recessive  factor  (b)  for  producing 
an  absolutely  white  color  and  also  the  recessive 
(a)  which  merely  " dilutes"  the  black  to  blue.  The 
"diluter"  a  of  course  can  not  have  any  visible  effect 


180  MULTIPLE    FACTORS 

in  a  mouse  already  carrying  b  and  therefore  white. 
There  are  also  reverse  cases  where,  in  the  presence  of 
B,  a  and  A  produce  no  different  effect  and  thus  a 
ratio  of  12AB  +  aB:3Ab:lab  is  obtained. 

Departures  from  the  9:3:3:1  ratio  different  from 
those  given  above  result  if  one  factor  for  a  character 
is  dominant  and  another  recessive.  For  example, 
there  is  a  white  race  of  fowls  that  is  dominant  and 
another  white  race  that  is  recessive.  There  are  two 
cocoon  colors  in  silkworm  moths  that  have  this  same 
relation.  A  cross  of  a  dominant  white  to  a  recessive 
white  gives  a  ratio  of  13:3.  Here,  instead  of  the 
recessive  classes  resembling  each  other,  so  that  a 
9 : 6 : 1  or  9 : 7  ratio  is  produced,  both  the  9AB  and  3Ab, 
since  they  contain  the  dominant  white  (A),  re- 
semble the  one  ab  containing  the  recessive  white 
(b),  and  only  the  3aB  appear  colored.  In  this 
case  the  effect  of  the  white  does  not  happen  to  be 
cumulative,  but  there  is  no  reason  why  factors  which 
differ  as  to  dominance  should  not  have  a  cumulative 
action;  if  they  did,  a  3  :10  :3  ratio  would  result. 

Cases  belonging  to  any  of  the  types  given  above 
show  modified  ratios  if  the  dominance  is  incomplete, 
for  then  the  heterozygous  classes  are  intermediate  in 
character  between  the  others.  Consequently,  in 
these  cases,  the  different  classes  are  usually  not 
as  easy  to  distinguish  from  one  another  as  if  domi- 
nance were  complete,  for  the  character  differences  now 
separating  the  classes  are  smaller.  In  such  cases, 
especially  if  the  character  is  appreciably  influenced 
by  environmental  conditions,  the  individuals  in  any 


MULTIPLE    FACTORS  181 

one  class  may  vary  so  much  from  each  other  as  to 
overstep  the  small  differences  separating  the  classes. 
An  accurate  separation  of  the  individuals  into  differ- 
ent classes  and  a  count  of  the  number  in  each  class  is 
then  impossible,  and  it  becomes  so  difficult  to  de- 
termine the  number  of  factors  involved  and  the 
effect  of  each  factor  (or,  rather,  factor-difference) 
that  such  cases  have  at  times  been  used  in  attempts 
to  disprove  the  factorial  hypothesis.  The  problem  is 
likewise  more  difficult  if  more  than  two  factor- 
differences  occur.  This  is  true  especially  in  those 
cases  where  the  effects  of  the  different  factors  are 
cumulative,  for  then  classes  are  produced  showing 
characters  intermediate  in  various  degrees  between 
the  characters  of  the  most  extreme  classes,  just  as  in 
cases  of  incomplete  dominance.  It  will  be  instructive 
to  consider  several  instances  of  crosses  of  the  above 
types,  since,  although  definite  ratios  can  not  be 
obtained,  there  are  various  characteristic  effects 
produced  which  show  that  multiple  factors  are  re- 
sponsible for  the  peculiarities  of  the  results. 

The  inheritance  of  black  color  in  Drosophila  has 
already  been  described.  Black  is  recessive  to  the 
normal  ("gray")  color,  but  the  heterozygous  forms 
are  a  little  darker  than  the  pure  grays.  Ebony  is 
another  body  color,  similar  in  appearance  to  black, 
but  somewhat  darker.  It  is  similarly  recessive  to 
gray,  but  the  factor  responsible  for  it  is  located  in  a 
different  chromosome  (III)  from  that  which  carries 
the  factor  for  black  (II).  When  black  and  ebony 
are  mated  together  we  should  expect  gray  flies  in  Fi. 


182  MULTIPLE    FACTORS 

Such  flies  were  actually  obtained,  although  they  were 
rather  dark  in  color,  since  both  black  and  ebony 
produce  some  effects  on  flies  hetenm-gous  for  them. 
In  F2  the  expectation  is  9  gray,  3  black,  3  ebony, 
and  1  black  ebony  (double  recessive).  When  F2 
was  actually  obtained  it  was  found  to  be  impossible 
to  make  an  accurate  separation  of  the  four  classes. 
There  was  a  practically  complete  series  ranging  from 
the  normal  gray  to  individuals  darker  than  either 
black  or  ebony.  The  gradation  is  obviously  due 
chiefly  to  the  fact  that  dominance  is  not  complete. 
There  are  nine  different  classes  expected,  instead  of 
four,  if  heterozygous  forms  be  counted.  These  nine 
classes  form  groups,  each  with  its  own  mode,  the 
outlying  members  of  each  group  overlapping  neigh- 
boring groups.  To  add  to ,  the  difficulty,  the  colors 
change  considerably  writh  the  age  of  the  fly.  There 
are  at  least  seven  other  mutant  factors  known  in 
Drosophila  that  make  the  flies  darker.  It  will 
readily  be  seen  that,  if  one  had  a  population  contain- 
ing a  mixture  of  all  these  characters,  analysis  would  be 
well-nigh  impossible. 

Before  making  the  above  cross  the  inheritance  of 
black  and  of  ebony  had  been  studied  separately,  and 
no  difficulty  in  classification  is  encountered  unless 
they  are  used  in  the  same  cross.  This  information 
made  it  possible  for  us  to  interpret  the  black  ebony 
cross.  In  the  experiments  now  to  be  described,  we 
are  dealing  with  factors  which  had  not  first  been 
studied  separately,  so  that  the  interpretation  is  not  so 
obvious  as  in  the  preceding  case. 


MULTIPLE    FACTORS       ,  183 

Two  varieties  of  tobacco,  Nicotiana  alata  grandi- 
flora  and  N.  forgetiana,  were  crossed  by  East.  They 
differ  mainly  in  the  size  and  color  of  the  flower.  The 
corolla  is  three  times  as  long  in  one  as  in  the  other 
variety,  as  seen  in  Fig.  55.  In  the  table,  page 
185,  the  lengths  of  the  corolla  in  the  two  varieties, 


FIG.  55. — At  the  left  a  flower  of  Nicotiana  alata  grandiflora;  at  the 
right  a  flower  of  N.  forgetiana;  in  the  middle  the  Fi  hybrid.     (After 

East.) 

in  the  FI,  and  in  the  F2  plants  are  given.  The  table 
shows  the  small  variability  of  the  parents.  The  Fi 
generation  is  intermediate  in  length  and  also  shows 
little  variability,  while  the  F2  generation  gives  no 
definite  ratios  but  exhibits  great  variability  (Fig.  56), 
and  overlaps  the  two  grandparental  types,  although 
only  a  few  flowers  in  F2  are  identical  in  size  with  those 
of  each  of  the  two  grandparental  types.  These 
results  are  those  expected  if  the  two  parent  varieties 


184 


MULTIPLE    FACTORS 


differ  in  several  factors  that  affect  their  size.  If  the 
parent  strains  were  pure  the  FI  hybrids  would  all  be 
alike,  or  rather  would  show  little  if  any  more  varia- 
bility than  either  parent  stock,  because  all  these  FI 
plants  receive  the  same  contributions  from  the 


FIG.  56. — At  left,  a  flower  of  Nicotiana  alata  grandiflora;  at  right  X. 
forgetiana;  between  them  are  four  F2  flowers,  showing  the  result  of 
segregation  both  in  the  length  and  the  spread  of  the  corolla.  (After 
East.) 

parents.  But  when  in  the  gametogenesis  of  the  FI 
plants  these  factors  segregate,  many  new  combina- 
tions will  be  formed,  and  among  them  will  be  a  few 
combinations  like  those  in  the  original  varieties; 
hence  we  expect  in  the  F2  a  wider  variability,  with  a 
return  to  the  grandparental  types  in  a  certain  per- 
centage of  the  plants.  East  suggests  that  four 
pairs  of  factors  may  cover  the  results  in  this  instance. 


MULTIPLE    FACTORS 


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186 


MULTIPLE    FACTORS 


A  race  of  pigeons  called  fantails  differs  from  other 
pigeons,  and  from  birds  in  general,  by  the  large  num- 
ber of  feathers  in  the  tail.  The  ordinary  pigeons  have 


! 


;. 


L. 


Back   fross 

1 

i 

1 

i 

|     | 

F, 

i 

!  ,  •  , 

FIG.  57. — Illustrating  the  results  of  a  cross  between  pigeons  with  12 
tail  feathers  and  a  race  of  fantail  pigeons  with  from  28  to  38  tail  feathers. 
The  number  of  feathers  in  Pi,  Fi,  F2,  and  the  offspring  of  the  backcross 
(Fi  by  fantail)  is  given.  In  each  case  the  numbers  on  the  base  line  stand 
for  tail  feathers.  The  vertical  columns  are  the  classes. 

twelve  tail  feathers;  the  fantails  used  in  the  cross 
have  from  28  to  38  tail  feathers.  The  Fi  hybrids 
(Fig.  57)  have  from  12  to  20  tail  feathers;  the  F2  have 
12  to  25  as  also  shown  in  figure  57.  When  the  FI 


MULTIPLE    FACTORS  187 

birds  are  backcrossed  (Fig.  57)  to  the  fantail  the 
number  of  feathers  varies  from  19  to  31.  On  the 
hypothesis  that  the  race  of  fantails  has  been  built 
up  by  the  accumulation  of  several  factors  these  results 
can  be  understood. 

MacDowell  has  compared  the  length  of  skull  and 
of  one  of  the  bones  in  the  leg  (ulna)  of  hybrids  be- 
tween domesticated  races  of  rabbits  in  the  Fi 
generation  and  in  the  backcross.  As  shown  in  the 
table,  page  185,  the  variability  of  the  backcross  is  in 
both  characters  greater  than  that  of  FI.  Similar 
though  less  convincing  evidence  was  obtained  for 
body  weight  also. 

The  inheritance  of  ear  length  in  rabbits  has  been 
studied  by  Castle  in  a  cross  between  lop-eared  and 
short-eared  races  (Fig.  58).  He  shows  that  the  FI 
generation  has  ears  of  intermediate  length  and  that 
the  blend  is  "permanent,"  i.e.,  that  "no  reappearance 
of  the  grandparental  ear  length  occurs  in  generation 
F2,  nor  are  the  individuals  of  the  second  generation, 
as  a  rule,  more  variable  than  those  of  the  first 
generation  of  cross  breeds."  In  the  light  of  Mac- 
Dowell's  results  for  other  quantitative  characters  in 
rabbits  it  seems  more  probable  that  the  number  of 
factors  involved  is  greater  for  ear  length  than  in  the 
other  cases,  hence  more  data  will  be  necessary  before 
we  can  be  certain  that  no  reappearance  of  the 
grandparental  types  will  be  found  in  F2.  If  four 
independent  factors  were  involved  either  grand- 
parental  type  would  be  expected  to  reappear  only 
once  in  256  times,  with  six  factors  only  once  in  4000 


188 


MULTIPLE    FACTORS 


times,  etc.  It  would  require  a  large  number  of  off- 
spring to  prove  the  multiple  factor  hypothesis  if  the 
reappearance  of  the  grandparental  types  be  de- 
manded for  such  a  proof. 


F 
t 


F 

z 


FIG.  58. — Short-eared  by  lop-eared  rabbit.     FI,  son  of  last;  F2.  daughter 
of  FI  by  his  sister.     (After  Castle.) 

Several  excellent  cases  of  multiple  factors  have  been 
worked  out  with  Indian  corn.  Height  of  plant 
(as  a  concomitant  of  its  vigor),  length  of  ear,  and 


MULTIPLE    FACTORS 


189 


IS         ti         !S        16 


FIG.  59.— Top  line;  at  left,  Tom  Thumb  pop  corn;  at  right,  black 
Mexican  sweet  com.  Middle  row;  Fi  from  crossing  the  above  races. 
Lower  line  F2  of  same  cross.  (After  East.) 


190  MULTIPLE    FACTORS 

productivity  depend  on  multiple  factors.  For  ex- 
ample, East  crossed  the  strain  Tom  Thumb  (having 
short  ears)  to  black  Mexican  sweet  (having  long  ears) . 
The  relative  length  of  ear  in  these  two  races  is  shown 
in  the  upper  line  of  Fig.  59,  to  the  left  and  to  the 
right.  A  sample  of  the  Fi  ears  is  shown  in  Fig  59, 
the  middle  of  the  figure,  while  the  variability  of  the 
F2  ears  is  shown  in  the  lowest  line.  It  is  evident  not 
only  that  the  original  types  reappear,  but  that  there 
are  all  intermediate  lengths  of  ear  in  F2. 

Many  cases  like  this  one  that  show  a  small  varia- 
bility in  Fi  and  a  greater  variability  in  F2  have  been 
described,  for  example,  in  oats  (Nilsson-Ehle),  beets 
(Kajanus),  turnips  (Kajanus),  barley  (Johannsen), 
gourd  (Emerson),  flax  (Tammes),  tobacco  (Hayes 
and  East),  evening  primrose  (Heribert-Nilsson), 
bean  (Emerson,  Johannsen),  pea  (Tschermak),  Lyon 
bean  (Belling),  wheat  (Nilsson-Ehle),  corn  (East, 
Emerson,  Hayes),  duck  (Phillips),  fowl  (Pearl), 
man  (Davenport),  rabbit  (Castle,  MacDowell),  mouse 
(Cuenot),  rat  (Castle,  Hagedoorn).1 

This  partial  list  will  serve  to  show  how  often  this 
form  of  inheritance  has  been  met  with,  and  when  it  is 
stated  that  in  a  number  of  these  plants  or  animals 
several  characteristics  show  this  kind  of  inheritance, 
its  frequency  will  be  apparent.  Many  but  not  all 
of  these  cases  relate  to  size,  and  size  is  obviously  a 
character  toward  which  many  separate  parts  con- 
tribute. Moreover  size  is  often  an  important  element 
in  domesticated  animals  and  plants,  and  any  differ- 

1  This  list  is  an  abbreviation  of  the  one  compiled  by  G.  H.  Shull. 


MULTIPLE    FACTORS  191 

ences  in  size  that  appear  might  therefore  be  selected 
in  order  to  produce  new  and  larger  strains. 

A  more  difficult  case  than  those  given  above  is  that 
of  truncate  (Fig.  18,  b)  in  Drosophila,  worked  out  by 
E.  R.  Altenburg  and  H.  J.  Muller.  The  F2  resulting 
from  a  cross  of  a  truncate  fly  to  a  normal  long-winged 
fly  consists  of  85-92  per  cent,  of  long-winged  and  the 
rest  truncates  and  flies  with  wings  of  various  inter- 
mediate grades.  The  extracted  truncates  do  not 
breed  true;  by  selection  it  is  possible  gradually  to 
reduce  the  longs  to  about  5  per  cent.,  but  even  after 
about  100  generations  of  selection  the  proportion  of 
longs  could  not  be  reduced  any  further.  These  longs 
produce  some  truncates,  but  do  not,  on  the  whole, 
produce  nearly  as  high  a  percentage  of  them  as  do 
their  truncate  brothers  and  sisters.  The  longs, 
therefore,  differ  genetically  from  the  truncates,  and 
the  fact  that  these  genetic  differences  are  constantly 
occurring  in  this  stock,  in  spite  of  the  long-continued 
selection,  seemed  to  indicate  that  here  at  least  there 
was  a  case  of  instability  of  factors  or  contamination 
of  allelomorphs. 

By  means  of  linkage  experiments  it  was  shown  that 
in  the  production  of  this  character  there  are  involved 
at  least  three  factors  (Ti,T2,T3),  one  in  the  first,  one 
in  the  second,  and  one  in  the  third  chromosome. 
The  character  cannot  make  its  appearance  without 
the  factor  in  the  second  chromosome  (T2),  but  it 
may  appear  without  either  of  the  other  two  factors, 
which  are,  therefore,  in  the  nature  of  intensifiers. 
Moreover,  truncate  is  influenced  by  still  other  fac- 


192  MULTIPLE    FACTORS 

tors.  For  instance,  bar,  a  first  chromosome  factor, 
acts  in  much  the  same  way  as  the  ordinary  first  chro- 
mosome intensifier.  The  sex  factor  also  intensifies 
truncate,  i.e.,  truncate  appears  more  readily  in  the 
females  than  in  the  males  and  may,  therefore,  be  called 
partially  "sex  limited."  Especially  noteworthy  is 
the  fact  that  while  recessive  in  the  normal  gray  it  is 
generally  dominant  in  an  individual  either  homozyg- 
ous  or  heterozygous  for  black. 

This  latter  circumstance  made  it  possible  to  study 
truncate  as  a  dominant  in  heterozygous  condition. 
As  will  appear  later,  this  simplified  the  problem 
greatly,  especially  in  determining  whether  or  not 
(1)  the  factors  for  truncate  are  stable;  (2)  whether 
they  are  contaminated  by  their  allelomorphs. 

A  truncate  male  containing  factors  for  truncate  in 
both  its  second  and  third  chromosomes  was  mated 
to  a  normal  winged  female  containing  in  its  second 
chromosomes  the  factor  for  black,  and  in  its  third 
chromosomes  the  factor  for  pink.  The  male  offspring 
of  this  mating  will,  therefore,  have  the  formula 

T2  gray        T3  red  . 

i—  T,  i  ,—  .  ,  .  1  hey  will  not  contain  1 1 ,  as 
long  black  long  pink 

males  derive  all  sex-linked  factors  from  their  mother. 
An  FI  male  was  then  backcrossed  to  black  pink 
females.  Since  there  is  no  crossing  over  in  the  male, 
all  the  gray  red  offspring  of  this  backcross  will  be 
genetically  identical,  and  like  their  father — unless 
the  factors  for  truncate  are  unstable,  or  contaminated 
by  their  normal  allelomorphs.  The  gray  reds  were 
not  all  alike  in  appearance,  however,  some  being 


MULTIPLE    FACTORS  193 

truncate,  though  most  were  long.  Males  of  these  two 
classes  were  then  mated  individually,  again  to  black 
pink  females.  From  the  result  of  these  matings  it 
was  clearly  shown  that  the  longs  and  the  truncates 
produced  almost  exactly  the  same  proportion  of 
truncate,  proving  that  they  were  alike  genetically. 
Moreover,  continuous  selection  of  males  of  this  com- 
position for  many  generations  in  an  attempt  to  alter 
this  ratio  was  without  effect.  Since  such  an  altera- 
tion did  not  occur  after  many  generations  of  out- 
crossing  (heterozygosis)  there  could  not  have  been 
any  contamination  or  miscibility  of  the  truncate 
factors  with  their  allelomorphs,  nor  any  instability 
of  these  factors. 

It  will  be  recalled  that  in  the  truncate  stock  there 
is  a  true  genetic  difference  between  the  long-winged 
and  the  truncate  flies,  but  since  it  has  been  shown 
that  the  truncate  factors  themselves  do  not  vary, 
this  genetic  variation  that  is  continually  occurring  in 
truncate  stock  must,  therefore,  be  due  to  the  fact  that 
flies  homozygous  for  a  large  number  of  the  factors 
favoring  the  appearance  of  truncate  are  either  not 
viable  or  else  infertile,  and  consequently  a  pure  stock 
cannot  be  maintained.1  In  support  of  the  latter 
explanation  it  is  found  that  the  greater  the  percentage 
of  truncate  produced  by  a  stock  the  lower  its  fertility. 

This  case  is  of  interest  not  only  because  the  results 
indicate  that  other  non-conformable  instances  might 

1  Nevertheless  stock  can  be  maintained  by  the  method  of  repeated 
backcrossing  to  black  pink,  given  above,  from  which  individuals  of  a 
definite,  known  composition  can  always  be  obtained. 


194 


MULTIPLE   FACTORS 


be  similarly  explained,  but  also  because  the  new 
methods  which  have  been  developed  in  attacking  it 
are  singularly  adapted  to  the  solution  of  such  prob- 
lems. The  use  of  this  method  has  been  made  pos- 
sible by  the  information  at  hand  as  to  the  linkage 


FIG.  60. — Normal  wing  (to  left)  and  beaded  wing  (to  right)  of 
Drosophila. 


groups  and  as  to  non-crossing  over  in  the  male. 
Without  such  knowledge  the  case  would  have  been 
practically  insoluble. 

The  same  method  of  attack  has  also  been  used  by 
Dexter,  in  his  experiments  with  the  "beaded"  wing 
of  Drosophila  (Fig.  60).  The  beaded  character  is  a 
variable  one,  some  of  the  beaded  individuals  being 


MULTIPLE    FACTORS  195 

very  nearly  normal  in  appearance.  The  degree  of 
abnormality  and  the  proportion  of  abnormal  off- 
spring are  both  capable  of  being  altered,  within 
limits,  by  selection  or  by  crossing  to  normal  stock. 
Dexter  crossed  beaded  flies  to  flies  carrying  mutant 
factors  in  the  different  chromosomes  and  studied 
the  linkage  of  the  beaded  character  with  these  other 
characters.  He  found  that  beadedness  showed  link- 
age to  third  chromosome  characters,  indicating  that 
there  is  at  least  one  factor  for  the  character  located  in 
that  chromosome.  He  also  found  that  sometimes 
beadedness  showed  linkage  to  second  chromosome 
characters,  while  at  other  times  it  failed  to  do  so. 
This  indicates  that  the  beaded  stock  was  impure  for  a 
factor  located  in  the  second  chromosome,  which  when 
present  increases  the  amount  of  beading.  Selection 
would  be  effective  either  by  eliminating  or  by  pre- 
serving this  factor. 

An  extensive  selection  experiment  was  carried  out 
by  Lutz  on  Drosophila.  He  selected  for  abnormal 
wing  venation — chiefly  for  extra  veins.  Abnormali- 
ties occur  in  nature  in  about  0.3  per  cent,  of  the  flies. 
In  two  separate  experiments  Lutz  increased  this  to 
approximately  100  per  cent,  abnormals,  and  in  one 
of  the  experiments  kept  it  there  for  eight  generations. 
But,  in  this  same  experiment,  one  pair  (brother  and 
sister  of  the  first  pair  that  produced  100  per  cent. 
abnormals)  produced  no  abnormals,  and  their  de- 
scendants remained  for  40  generations  a  strain  which 
gave  scarcely  more  abnormals  than  does  a  wild 
strain.  Possibly  a  mutation  occurred  here,  although 


196 


MULTIPLE    FACTORS 


a  cross  between  this  " reverted"  strain  and  the  se- 
lected 100  per  cent,  abnormal  strain  failed  to  give  a 
definite  result.  From  the  offspring  of  this  cross, 
again  crossed  to  the  100  per  cent,  strain,  Lutz  selected 
another  abnormal  strain,  which  produced  from  95  to 
100  per  cent,  abnormals  for  eight  successive  genera- 
tions. He  then  selected  back  again  for  normals  and 
in  six  generations  he  obtained  a  strain  wiiich  produced 
no  more  abnormals  than  does  a  wild  strain.  Selec- 
tion for  reduced  veins  was  also  successful,  but  was  not 
carried  on  very  extensively. 


FIG.  61. — Series  of  arbitrary  grades  of  hooded  rats  used  in  classifying 
results  of  selection  experiment.  Above  the  figures  the  numbers  assigned 
to  the  grades  are  given  (see  text).  (After  Castle  and  Phillips.) 

It  is  not  clear  what  interpretation  should  be  placed 
upon  these  experiments  of  Lutz,  but  it  seems  probable 
that  mutations  affecting  the  venation  occurred 
several  times,  and  were  selected. 

One  of  the  most  exhaustively  studied  cases  of  the 
effect  of  selection  on  a  mixed  population  is  that  car- 
ried out  on  hooded  rats  by  Castle  and  his  co-workers, 
particularly  Phillips.  The  pattern  of  hooded  rats  is 


MULTIPLE    FACTORS  197 

shown  in  Fig.  61.  The  dark  pigment  covers  the 
head  and  extends  as  a  stripe  down  the  back.  The 
extent  of  the  hood  and  the  breadth  of  the  dorsal 
band  are  so  variable  that  in  one  direction,  called 
plus,  the  rat  is  all  black,  except  for  a  white  stripe  on 
the  belly,  and  in  the  other  direction,  minus,  the  only 
black  present  is  on  the  head. 

Two  selections  were  carried  out:  one  in  the  plus 
direction  (toward  the  darker  type),  the  other  in  the 
minus  direction  (toward  the  lighter  type).  The 
steady  progress  in  the  plus  direction  that  took  place 
during  13  generations  is  shown  on  page  98,  and  in 
the  minus  direction  on  page  199. 

This  progress  in  the  direction  of  selection  would  be 
expected  if  the  race  were  not  at  the  start  pure  for 
factors  that  determine  the  amount  of  pigmentation, 
since  in  all  such  cases  the  process  of  selection  in  a 
heterogeneous  population  sorts  out  some  of  the  fac- 
tors from  others.  Selection  in  most  cases  creates 
nothing  that  is  not  already  present,  but  separates 
existing  factors. 

There  are  several  ways  in  which  the  composition 
of  the  rats  after  their  selection  can  be  tested,  and  some 
of  these  tests  Castle  and  Phillips  have  made.  When 
light-colored  rats  from  the  minus  series  were  bred  to 
wild  or  to  Irish  rats  that  had  a  uniformly  (or  nearly 
uniformly)  dark  coat,  all  the  offspring  had  practically 
completely  colored  coats.  When  these  were  inbred 
they  gave  3  uniform  to  1  hooded  coat.  This  result 
shows  that  there  is  one  chief  factor  (which  is  re- 
cessive) for  hooded  coat.  However,  the  F2  hooded 


198 


MULTIPLE    FACTORS 


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°  ,CQ                        T-H    T-H    CO    t^-  CO    CO   t^-    *O    ^           OS    O    iO 

**s 

T — 

il      , 

o               :    :    :    :    :  :    :    :    :    :        :        :    H 

COTf»O  CO1>-OOOSO           i— i    CO    CO 

T-H  T-H      T-H      T— I 


200  MULTIPLE    FACTORS 

rats  differed  more  among  themselves  than  did  those 
from  the  grandparental  strain  of  hooded  rats,  which 
shows  that  other  factors  were  involved  as  well, 
that  modified  the  extent  of  pigmentation  of  the 
hooded  coat,  but  had  little  effect  on  the  uniform  coat. 
The  range  of  variation  was  extended  in  the  direction 
of  the  darker  coat,  showing  that  modifying  factors 
causing  a  darker  coat  had  been  introduced  from  the 
wild  strain;  and  such  would  be  the  expectation  if 
selection  had  eliminated  from  the  domesticated  strain 
some  of  the  factors  making  for  the  darker  coat  that 
had  been  present  in  the  original  impure  population. 
Conversely  the  darker  hooded  rats,  plus  series,  were 
bred  to  wTild  gray  rats:  the  FI  were  uniform;  these 
inbred  gave  3  uniform  to  1  hooded  in  F2.  The  range 
of  variation  of  the  latter  was  again  greater  than  that 
present  in  the  dark  hooded  rats  which  had  not  been 
outcrossed,  but  now  the  range  extended  rather  in  the 
minus  direction,  i.e.,  the  F2  hooded  rats  wrere  on  the 
whole  lighter  than  their  dark  hooded  grandparents. 
The  result  is  what  the  multiple  factor  hypothesis  calls 
for,  if  the  wild  or  Irish  rats  contain  factors  that  influ- 
ence the  condition  of  the  color  pattern.  Plus  selec- 
tion had  weeded  out  some  of  the  " minus"  factors,  but 
crossing  with  a  race  in  which  no  selection  had  been 
practised  brought  them  back.  When  the  selected 
plus  and  minus  races  were  crossed  to  each  other  the 
variability  was  somewhat  increased  in  FI,  and  was 
further  increased  in  F2.  The  extreme  conditions  of 
the  grandparents  rarely  appear  in  this  generation. 
Again  the  results  are  those  the  theory  calls  for. 


MULTIPLE    FACTORS  201 

The  test  of  reversing  the  direction  of  selection  was 
tried.  The  parents  belonged  to  the  6  (and  "6^") 
generation  of  the  minus  selection  series,  and  aver- 
aged —  1.86.  The  average  grade  of  the  offspring  was 
— 1.56,  a  regression  of  0.30,  and  their  range  was  from 
0  to  —2.50.  Some  of  the  low-grade  offspring  ranging 
from  —0.37  to  —0.87  were  chosen  for  the  return 
selection.  They  produced  118  offspring  whose  aver- 
age was  —1.28,  a  regression  of  0.68,  which  is  in  the 
opposite  direction  from  the  regression  obtained  in  the 
former  (minus)  selection.  For  six  generations  the 
reversed  selection  went  on  and  carried  the  race  back 
along  its  former  course,  i.e.,  toward  its  original  con- 
dition. The  fact  that  selection  in  the  original  direc- 
tion was  still  producing  some  effect  when  the  reversed 
selection  began,  means,  on  the  multiple  factor 
hypothesis,  that  the  stock  was  still  heterogeneous,  in 
some  factors  at  least,  and,  therefore,  reversing  the  proc- 
ess would  be  expected  to  give  the  results  that  Castle 
and  Phillips  obtained. 

These  important  results  of  Castle  and  Phillips  ful- 
fil so  entirely  the  expectation  for  multiple  factors 
that  they  might  have  been  utilized  as  a  good  illustra- 
tion of  the  effects  of  selection  on  a  group  in  which  a 
particular  character  owed  its  modifications  to  multi- 
ple factors.  Castle  has,  on  a  number  of  occasions, 
made  use  of  these  results  to  expound  a  very  different 
interpretation.  The  experiments  were  begun,  in  fact, 
to  see  whether  selection  in  a  given  direction  of  a 
varying  character  that  gave  a  continuous  series  of 
types  would  tend  to  further  variation  in  the  same 


202  MULTIPLE    FACTORS 

direction.  In  other  words  it  was  intended  to  discover 
whether  a  new  genetic  type,  with  a  new  mode,  could 
be  established  as  a  result  of  selection,  so  that  the 
original  bounds  of  variability  would  be  transgressed. 
Castle  has  interpreted  his  results  to  mean  that 
through  selection  or  after  selection,  a  unit  character 
can  be  changed.  He  has  used  at  times  a  word  fa- 
miliar to  readers  of  Darwin,  namely  "  potency."  The 
potency  of  a  factor  as  well  as  of  a  character  is  sup- 
posed to  be  a  somewhat  variable  element. 

It  is  obvious  that  it  would  be  exceedingly  difficult 
to  establish  such  an  interpretation,  because  in  order 
to  prove  that  selection  can  alter  a  factor  it  would  first 
be  necessary  to  prove  that  recombinations  of  multiple 
factors  were  not  responsible  for  the  variations  of  the 
"unit"  character.  The  results  with  rats  are  in  har- 
mony with  the  theory  of  multiple  factors,  and  hence 
in  harmony  with  the  whole  body  of  Mendelism. 
There  are  no  a  priori  grounds  for  regarding  quantita- 
tive factors  as  differing  from  other  Mendelian  factors, 
and  many  cases  are  knowTn  in  which  quantitative 
factors  conform  in  every  respect  to  Mendel's 
principles. 

In  support  of  the  view  that  the  particular  character 
of  the  hooded  rat  differs  from  the  wild  rat  by  a  single 
factor  Castle  has  pointed  out  that  this  is  established 
by  the  Mendelian  ratio,  3:1,  that  obtains  when  these 
types  are  crossed.  But  the  3 : 1  ratio  does  not  estab- 
lish this  view.  The  ratio  only  shows  that  a  recessive 
factor  for  hoodedness  must  be  present  in  order  that 
the  rats  may  be  hooded  at  all.  Other  factors  that 


MULTIPLE    FACTORS  203 

modify  the  coat  may  produce  a  visible  effect  only  in 
the  presence  of  this  chief  factor  for  hoodedness.  The 
F2  from  the  crosses  to  self-color  indicate  that  such 
modifiers  are  really  present  in  the  rats.  The  under- 
standing of  this  point  is  so  important  that  similar 
relations  of  the  same  sort  may  be  cited.  If  a  choco- 
late mouse  (i.e.,  one  that  carries  the  factors  for  black 
and  for  cinnamon)  is  mated  to  a  white  mouse  carry- 
ing the  factors  for  gray  (instead  of  those  for  black  and 
cinnamon)  the  Fi  generation  will  be  gray.  In  the 
F2  there  are  three  colored  mice  to  one  white  one,  but 
there  are  several  sorts  of  colored  mice.  Color  of  any 
kind  is  dependent  on  the  action  of  a  factor  allelo- 
morphic  to  white,  hence  the  3:1  ratio,  but  this  clas- 
sification ignores  the  occurrence  of  several  kinds  of 
colored  mice  which  are  due  to  differences  in  other 
factors  determining  what  kind  of  color  will  develop. 

There  is  a  case  in  Drosophila  that  illustrates  the 
same  point.  Eosin  is  a  light  eye  color.  Another 
factor  called  cream  produces  no  effect  on  other  eye 
colors,  but  makes  eosin  still  lighter.  A  male  pure  for 
cream  and  for  eosin  bred  to  a  red  female  gives  red 
eye  color  in  FI.  The  Fi's  inbred  give  three  reds  to 
one  light  eye  color,  but  among  the  lights  three 
different  but  overlapping  kinds  may  be  detected. 
Here,  as  in  Castle's  case,  there  is  a  chief  factor  (eosin) 
for  reduced  pigmentation,  which  must  be  present  if 
any  reduction  in  the  color  occurs  at  all,  and  another 
factor  (cream)  that  modifies  the  amount  of  pigmen- 
tation only  when  the  chief  factor  is  present. 

In  favor  of  the  view  that  factors  are  constant  are 


204 


MULTIPLE    FACTORS 


the  convincing  experiments  of  Johannsen  on  the  size 
of  the  Princess  beans.  The  material  is  highly  fav- 
orable for  work  of  this  kind,  not  only  because  exact 
measurements  may  be  taken,  but  because  the  stocks 


FIG.  62. — I.  Five  pure  lines  of  beans  (A,  B,  C,  D,  E),  and  the  popula- 
tion (A-E)  that  results  when  they  are  mixed.  II.  The  upper  figure  repre- 
sents the  original  biotype,  and  the  two  figures  below  this,  the  two  new 
biotypes  that  arose  from  it.  (After  Johannsen.) 

reproduce  by  self-fertilization  and  were  found  to  be 
homozygous.  Johannsen's  results  (Fig.  62)  show 
that  no  matter  how  many  factors  influence  the  size 
of  the  bean,  so  long  as  the  bean  is  homozygous,  selec- 
tion of  plus  and  minus  variants  produces  no  effect  on 


MULTIPLE    FACTORS  205 

subsequent  generations.  Exactly  the  opposite  results 
are  expected  when  the  population  is  heterogeneous  for 
multiple  factors  at  the  beginning. 

On  several  occasions  Castle  has  stated  that  the  prac- 
tical breeder  is  especially  familiar  with  the  effects  of 
selection  because  he  has  obtained  most  of  his  results  by 
this  method.  It  is  intimated  not  only  that  the  breeder 
is  in  a  position  to  look  favorably  on  the  doctrine  of  po- 
tencies, but  that  his  familiarity  with  work  of  selec- 
tion entitles  his  views  to  special  consideration.  But 
no  one  has  in  recent  years  denied  that  selection  of 
mixed  material  will  lead  to  the  isolation  of  definite 
types  and  even  of  new  types. 

To  what  has  been  said  one  additional  consideration 
must  be  urged.  Mutations  may  occur  at  any  time 
and  will  be  quickly  observed  if  they  are  in  the  direc- 
tion in  which  a  selective  process  is  being  carried  out. 
It  may  not  be  easy  to  recognize  the  first  appearance 
of  a  mutant  and,  in  fact,  its  presence  may  be  detected 
only  after  the  selection  has  gone  so  far  that  its  origin 
is  lost.  The  breeder  may,  if  he  is  not  extremely 
observant,  infer  that  his  selection  is  producing  the 
desired  effect  on  the  potency  of  the  character,  while 
in  reality  he  is  studying  the  influence  of  a  new 
factor  on  the  character  under  selection.  This  possi- 
bility may  be  illustrated  by  two  cases.  In  Castle's 
experiments  two  rats  appeared  that  behaved  like  a 
new  type.  In  fact  he  gives  them  the  value  of  mutants. 
In  Drosophila,  Morgan  carried  out  a  selection  experi- 
ment for  three  years,  involving  upward  of  75  gen- 
erations. The  character  selected  was  a  dark  "  trident " 


206 


MULTIPLE    FACTORS 


on  the  thorax  (Fig.  63).  In  a  few  generations  a 
minus  stock  with  no  trident  was  established  that  bred 
true.  The  plus  stock  went  up  and  down,  the  selec- 
tion being  not  always  thorough.  A  stock  that  always 
had  the  trident  present  to  some  degree  was  obtained 


FIG.  63. — Thorax  of  mutant  stocks  of  Drosophila  ampelophila.  a, 
race  "without"  trident;  b,  race  "with"  trident;  c,  race  called  streak; 
d,  race  called  trefoil;  e,  race  called  band. 

after  a  time.  Later  several  other  mutations  appeared, 
some  of  which  greatly  increased  the  black  on  the 
thorax ;  some  even  swamped  the  trident,  making  it  a 
broad  band.  Three  such  mutant  stocks  were  readily 
isolated.  It  might  have  been  concluded  that  these 
mutations  had  occurred  in  the  direction  of  selection, 


MULTIPLE    FACTORS  207 

because  selection  had  changed  the  potency  of  the 
trident  factor,  were  it  not  that  during  these  three 
years  over  100  other  mutant  characters  had  appeared 
in  Drosophila,  affecting  every  part  of  the  body.  Ob- 
viously when  such  changes  are  taking  place  every- 
where, one  would  almost  certainly  find  changes 
occurring  in  the  parts  that  were  being  carefully 
scrutinized  for  any  changes  whatever. 


CHAPTER  IX 
THE  FACTORIAL  HYPOTHESIS 

In  Mendelian  heredity  the  word  " factor"  is  used 
for  something  which  segregates  in  the  germ  cells, 
and  which  is  somehow  connected  with  particular 
effects  on  the  organism  that  contains  it.  For  exam- 
ple, if  a  fly  (?)  with  red  eyes  is  crossed  to  a  fly  (<*•) 
with  white  eyes,  there  will  be  in  F2  three  reds  to  one 
white,  and  this  ratio  can  be  explained  by  the  assump- 
tion that  in  the  Fi  hybrid  something  for  red  eyes 
has  separated  from  something  for  white  eyes. 

We  may  express  these  factorial  relations  in  another 
way  by  saying  that  a  germ  cell  that  produces  white 
eyes  differs  from  a  germ  cell  that  produces  red  eyes 
by  one  factor-difference.  We  think  of  this  difference 
as  having  arisen  through  a  factor  in  the  red-eyed  wild 
fly  mutating  to  a  factor  for  white. 

Mendelian  heredity  has  taught  us  that  the  germ 
cells  must  contain  many  factors  that  affect  the  same 
character.  Red  eye  color  in  Drosophila,  for  exam- 
ple, must  be  due  to  a  large  number  of  factors,  for  as 
many  as  25  mutations  for  eye  color  at  different  loci 
have  already  come  to  light.  Each  produced  a  specific 
effect  on  eye  color;  it  is  more  than  probable  that  in 
the  wild  fly  all  or  many  of  the  normal  allelomorphs  at 
these  loci  have  something  to  do  with  red  eye  color. 

208 


THE    FACTORIAL    HYPOTHESIS  209 

One  can  therefore  easily  imagine  that  when  one  of 
these  25  factors  changes,  a  different  end  result  is 
produced,  such  as  pink  eyes,  or  vermilion  eyes,  or 
white  eyes  or  eosin  eyes.  Each  such  color  may  be 
the  product  of  25  factors  (probably  of  many  more) 
and  each  set  of  25  or  more  differs  from  the  normal 
in  a  different  factor.  It  is  this  one  different  factor 
that  we  regard  as  the  "unit  factor''  for  this  particular 
effect,  but  obviously  it  is  only  one  of  the  25  unit 
factors  that  are  producing  the  effect.  However  since 
it  is  only  this  one  factor  and  not  all  25  which  causes 
the  difference  between  this  particular  eye  color  and 
the  normal,  we  get  simple  Mendelian  segregation  in 
respect  to  this  difference.  In  this  sense  we  may  say 
that  a  particular  factor  (p)  is  the  cause  of  pink,  for  we 
use  cause  here  in  the  sense  in  which  science  always 
uses  this  expression,  namely,  to  mean  that  a  particu- 
lar system  differs  from  another  system  only  in  one 
special  factor. 

The  converse  relation  is  also  true,  namely,  that  a 
single  factor  may  affect  more  than  one  character. 
For  example,  the  factor  for  rudimentary  wings  in 
Drosophila  affects  not  only  the  wings,  but  the  legs, 
the  number  of  eggs  laid,  the  viability,  etc.  Indeed, 
in  his  definition  of  mutation,  DeVries  supposed  that  a 
change  in  a  unit  factor  involves  all  parts  of  the  body. 
The  germ  cells  may  be  thought  of  as  a  mixture  of 
many  chemical  substances,  some  of  them  more  closely 
related  to  the  production  of  a  special  character,  color, 
for  example,  than  are  others.  If  any  one  of  the  sub- 
stances undergoes  a  change,  however  slight,  the  end 


210  THE    FACTORIAL    HYPOTHESIS 

product  of  the  activity  of  the  germ  cell  may  be 
different.  All  sorts  of  characters  might  be  affected 
by  the  change,  but  certain  parts  might  be  more  con- 
spicuously changed  than  are  others.  It  is  these  more 
obvious  effects  that  we  seize  upon  and  call  unit 
characters.  It  is  the  custom  of  most  writers  to  speak 
of  the  most  affected  part  as  a  "unit  character/'  and 
to  disregard  minor  or  less  obvious  changes  in  other 
parts.  They  frequently  speak  of  a  unit  character  as 
the  result  of  a  unit  factor,  forgetting  that  the  unit 
character  may  be  only  one  effect  of  the  factor. 

Failure  to  realize  the  importance  of  these  two 
points,  namely,  that  a  single  factor  may  have  sev- 
eral effects,  and  that  a  single  character  may  depend 
on  many  factors,  has  led  to  much  confusion  between 
factors  and  characters,  and  at  times  to  the  abuse  of 
the  term  "unit  character."  It  can  not,  therefore, 
be  too  strongly  insisted  upon  that  the  real  unit  in 
heredity  is  the  factor,  while  the  character  is  the  prod- 
uct of  a  number  of  genetic  factors  and  of  environ- 
mental conditions.  The  character  behaves  as  a  unit 
only  when  the  contrasted  individuals  differ  in  regard 
to  a  single  genetic  factor,  and  only  in  this  case  may 
it  be  called  a  unit  character.  As  soon  as  the  indi- 
viduals differ  by  two  or  more  genetic  factors  that 
affect  the  same  character  the  latter  can  be  no  longer 
considered  a  unit.  So  much  misunderstanding  has 
arisen  among  geneticists  themselves  through  the 
careless  use  of  the  term  "unit  character"  that 
the  term  deserves  the  disrepute  into  which  it  is 
falling. 


THE    FACTORIAL    HYPOTHESIS  211 

In  the  following  sections,  several  of  the  more  im- 
portant misconceptions  arising  from  the  confusion 
between  factors  and  characters  will  be  considered 
in  turn: 

1.  There  is  a  curious  objection  to  the  factorial 
hypothesis  that  is  sometimes  brought  forward.  It 
originated  apparently  as  an  objection  to  Weismann's 
idea  that  a  single  determinant  stands  for  a  single 
character.  Weismann's  idea  of  a  sorting  out  of 
determinants  undoubtedly  implies  something  of  this 
kind.  The  objection  states  that  the  organism  is  a 
whole — that  the  whole  determines  the  nature  of  the 
parts.  Such  a  statement,  in  so  far  as  it  has  any 
meaning  at  all,  rests  on  a  confusion  of  ideas.  That 
the  different  regions  of  the  developing  embryo  do 
sometimes  have  an  immediate  influence  on  each  other 
has  been  abundantly  demonstrated,  as  well  as  the 
fact  that  in  other  cases  parts  have  little  or  no  in- 
fluence on  each  other.  That  substances  are  pro- 
duced in  one  place  whose  principal  effects  are  seen 
in  other  places  is  not  likely  to  be  denied.  It  has 
even  been  insisted  in  the  preceding  pages  that  the 
evidence  from  heredity  indicates  with  great  proba- 
bility that  there  are  many  factors  whose  combined 
effect  is  necessary  for  the  production  of  each  separate 
character,  as  in  the  production  of  eye  color,  for 
example.  There  is  no  reason  why  this  interaction 
should  always  take  place  within  the  separate  cells; 
in  other  words,  why  the  products  of  factor  A  in  one 
cell  should  not  sometimes  affect  the  products  of 
factor  B  in  another  cell.  The  factorial  hypothesis 


212  THE    FACTORIAL    HYPOTHESIS 

does  not  assume  that  any  one  factor  produces  a 
particular  character  directly  and  by  itself,  but  only 
that  a  character  in  one  organism  may  differ  from  a 
character  in  another  because  the  sets  of  factors  in  the 
two  organisms  have  one  difference.  This  point  is 
not  likely  to  be  misunderstood  by  any  one  who  grasps 
the  meaning  of  the  factorial  hypothesis.  The  "or- 
ganism-as-a- whole "  argument,  so  long  as  it  is  not  a 
vague  and  mystical  sentiment  incapable  of  clear 
expression,  has  no  terrors  for  the  factorial  hypothesis, 
for  this  hypothesis  disclaims  any  intention  of  making 
one  unit  character  the  sole  product  of  one  factor  of 
the  germ. 

2.  No  one  disputes  that  characters  vary,  but  it  has 
become  necessary  to  explain  what  we  mean  by  this 
statement.  Many  populations  have  been  shown  to 
be  mixtures  of  different  genetic  types.  This  means 
that  many  of  the  individuals  have  different  germ 
plasms.  In  man,  for  instance,  there  are  blue-eyed, 
brown-eyed,  black-eyed  and  pink-eyed  individuals, 
and  these  variations  of  eye  color  have  been  shown 
by  Hurst,  the  Davenports,  Holmes  and  others  to 
depend  on  different  factorial  constitutions.  It  has 
been  shown  in  several  cases,  notably  in  corn,  by 
Shull,  and  by  East  and  Hayes,  that  populations  may 
contain  differences  in  many  factors  that  have 
similar  effects  on  the  same  character.  In  this  case 
too  the  different  factors  that  affect  a  part  in  the  same 
way  are  shown  to  separate  and  recombine  in  succes- 
sive generations.  The  result  is  variability,  but 
variability  of  a  sort  that  is  compatible  with  the 


THE    FACTORIAL    HYPOTHESIS  213 

invariability  of  the  factors  involved.  When,  how- 
ever, these  factors  were  sorted  out  so  that  strains 
became  homozygous,  some  variability  probably  due 
to  evironic  differences  still  remained.  That  is,  in 
addition  to  the  variation  due  to  recombination  it 
has  been  found  that  even  in  pure  races  "unit  char- 
acters" vary.  Why,  then,  it  may  be  asked,  do  not 
the  factors  that  produce  them  vary  also? 

Johannsen's  work  on  material  of  a  kind  suitable 
to  give  a  definite  answer  to  this  question  and  by 
methods  that  have  not  been  questioned,  has  brought 
out  clearly  certain  facts  only  vaguely  stated  before. 
In  a  population  of  beans  he  found  that  each  bean 
gave  rise  by  self-fertilization  to  what  he  called  a  pure 
line.  Each  of  the  original  beans  proved  to  be  homo- 
zygous for  all  of  the  factors  involved.  This  was 
probably  due  to  self-fertilization  through  many  genera- 
tions, a  process  that  automatically  produces  homo- 
zygous lines.  The  weights  of  the  descendants  of  any 
given  bean  gave  a  curve  of  frequency  which  was 
different  from  that  of  the  whole  population  (Fig.  62) . 
Within  the  group  derived  from  one  bean,  however,  it 
was  found  that  any  bean,  whether  heavier  or  lighter 
than  the  others,  gave  a  curve  exactly  like  the  curve 
of  the  line  from  which  it  came.  Evidently  then  the 
size  differences  within  these  pure  lines  are  not  inherited. 
They  must  be  due  to  the  environment  of  the  plant,  or 
to  the  position  of  the  bean  in  the  pod,  etc.;  in  other 
words  to  conditions  that  are  extrinsic  to  the  germ 
plasm.  Here  is  a  demonstration  that  the  factors 
do  not  vary,  but  give  identical  results  in  successive 


214          THE  FACTORIAL  HYPOTHESIS 

generations.     Of  course  this  demonstration  could  not 
have  been  made  with  heterozygous  individuals. 

3.  It  has  also  been  suggested  that  one  factor  may 
sometimes   contaminate  its  allelomorph,   when   the 
two  meet  in  the  hybrid.     There  is  no  a  priori  reason 
why  this  might  not  occur  so  far  as  we  can  see.     The 
question  is  whether  there  is  any  evidence  to  establish 
or  even  make  probable  such  a  view.     The  great 
body  of  Mendelian  evidence   points  unmistakably 
to  the  conclusion  that  as  a  rule  contamination  does 
not  occur.     It  will  require  equally  clear  evidence  to 
show  that  contamination  does  sometimes  take  place. 
Until  this  evidence  is  forthcoming  the  facts  which 
have  been  said  to  support  the  hypothesis  of  contami- 
nation find  a  more  consistent  explanation  on   the 
hypothesis  of  multiple  factors. 

4.  Bateson  has  recently  argued  from  the  visible 
differences   between   characters   that   a   process   of 
fractionation  of  factors  takes  place.     The  argument 
is  given  in  the  following  quotation: 

"Some  of  my  Mendelian  colleagues  have  spoken 
of  genetic  factors  as  permanent  and  indestructible. 
Relative  permanence  in  a  sense  they  have,  for  they 
commonly  come  out  unchanged  after  segregation. 
But  I  am  satisfied  that  they  may  occasionally  undergo 
a  quantitative  disintegration,  with  the  consequence 
that  varieties  are  produced  intermediate  between  the 
integral  varieties  from  which  they  were  derived. 
These  disintegrated  conditions  I  have  spoken  of  as 
subtraction — or  reduction — stages.  For  example, 
the  Picotee  sweet  pea,  with  its  purple  edges,  can  surely 


THE    FACTORIAL.  HYPOTHESIS  215 

be  nothing  but  a  condition  produced  by  the  factor 
which  ordinarily  makes  the  fully  purple  flower,  quanti- 
tatively diminished.  The  pied  animal,  such  as  the 
Dutch  rabbit,  must  similarly  be  regarded  as  the  re- 
sult of  partial  defect  of  the  chromogen  from  which  the 
pigment  is  formed,  or  conceivably  of  the  factor  which 
effects  its  oxidation.  On  such  lines  I  think  we  may 
with  great  confidence  interpret  all  those  intergrading 
forms  which  breed  true  and  are  not  produced  by 
factorial  interference. 

"It  is  to  be  inferred  that  these  fractional  degrada- 
tions are  the  consequences  of  irregularities  in  segrega- 
tion. We  constantly  see  irregularities  in  the  ordinary 
meristic  processes,  and  in  the  distribution  of  somatic 
differentiation.  We  are  familiar  with  half  seg- 
ments, with  imperfect  twinning,  with  leaves  partially 
petaloid,  with  petals  partially  sepaloid.  All  these 
are  evidences  of  departures  from  the  normal  regu- 
larity in  the  rhythms  of  repetition,  or  in  those  waves 
of  differentiation  by  which  the  qualities  are  sorted 
out  among  the  parts  of  the  body.  Similarly,  when 
in  segregation  the  qualities  are  sorted  out  among  the 
germ  cells  in  certain  critical  cell  divisions  we  can  not 
expect  these  differentiating  divisions  to  be  exempt 
from  the  imperfections  and  irregularities  which  are 
found  in  all  the  grosser  divisions  that  we  can  observe." 

Bateson  has  assumed  because  the  character  ap- 
pears to  fractionate  that  we  are  to  infer  that  some 
particular  factor,  that  stands  for  it,  fractionates  too, 
but  such  a  conclusion  overlooks  the  fact  that  a  char- 
acter is  produced  by  many  factors  in  co-operation, 


216         THE  FACTORIAL  HYPOTHESIS 

and  that,  in  consequence,  many  factor  differences 
may  occur  which  will,  in  turn,  cause  the  character 
differences  in  question.  Secondly,  Bateson  argues 
that  we  should  expect  these  irregularities  to  occur  in 
the  segregation  of  character-factors  during  germ-cell 
formation,  because  we  find  irregularities  in  the  seg- 
regation of  factors  during  development.  Appar- 
ently Bateson  holds  the  view  that  differentiation 
of  characters  is  the  result  of  sorting  out  of  factors 
in  the  somatic  divisions;  in  other  words,  he  adopts 
Weismann's  theory  of  embryonic  development.  Lo- 
calization of  factors  is  inferred  from  localization  of 
characters.  Hence  his  employment  of  the  idea 
chiefly  when  patterns  are  involved.  The  conclusion 
to  which  most  modern  students  of  experimental 
embryology  have  arrived,  a  conclusion  based  on  a 
considerable  body  of  evidence,  is  that  differentiation 
is  not  a  consequence  of  sorting  out  of  the  hereditary 
(genetic)  materials.  This  conclusion  is  not  con- 
sidered or  else  is  ignored  by  Bateson  in  this  argument. 
5.  The  confusion  of  character  with  factor  is  nowhere 
more  apparent  than  in  the  well-known  presence  and 
absence  hypothesis,  and  since  this  hypothesis  has 
been  so  widely  employed  in  Mendelian  literature  it 
calls  for  somewhat  more  extended  analysis.  The 
hypothesis  was  first  proposed  to  explain  the  inherit- 
ance of  combs  in  poultry  (Fig.  64).  Rose  comb  by 
single  comb  gives  in  F2  three  rose  to  one  single;  pea 
comb  to  single  gives  in  F2  three  pea  to  one  single. 
When  rose  is  bred  to  pea  a  new  type  of  comb,  called 
walnut,  appears,  and  in  F2  there  are  nine  walnut: 


THE    FACTORIAL    HYPOTHESIS 


217 


three  rose:  three  pea:  one  single.     Since  single  comb 
was  not  present  in  either  of  the  grandparental  strains, 


FIG.  64. — Combs  of  fowls,     a,  Single;  b,  pea;  c,  rose;  d,  walnut;  e,  Breda. 


how  then  can  its  appearance  in  this  cross  be  explained? 
The  difficulty  was  met  as  follows:    The  ratio  shows 


218  THE    FACTORIAL    HYPOTHESIS 

clearly  that  two  pairs  of  Mendelian  factors  are  present. 
Pea  comb  was  assumed  to  lack  a  factor  for  rose,  and 
rose  was  assumed  to  lack  a  factor  for  pea.  By  re- 
combination there  should  result  in  F2  one  individual 
in  sixteen  that  was  no-rose  no-pea.  This  is  the  single 
comb.  A  single  letter  or  symbol  S  was  inserted  in 
all  of  the  formula?  so  that  when  neither  rose  nor  pea 
comb  was  present  something  would  seem  to  be  left 
to  represent  the  single  comb. 

The  verification  of  the  latter  point  was  supposed 
to  be  found  in  the  relation  of  the  single  comb  to  a 
combless  condition  found  in  the  Breda  race  of  fowls, 
which,  when  crossed  to  single,  gave  in  F2  three 
singles  to  one  combless.  In  other  words  the  comb- 
less  fowl  was  supposed  to  represent  a  race  in  which  the 
lowest  stage  of  the  series  had  been  reached  and  the 
last  factor  for  comb  had  been  lost.  The  series  just 
described  was  represented  on  the  presence  and  ab- 
sence scheme  as  follows: 

Rose       RpS 
Pea         rPS 
Walnut  RPS 
Single     rpS 

There  is,  obviously,  no  necessity  to  make  these 
characters  depend  for  their  expression  on  losses  of 
something;  for  the  small  letters  that  here  stand  for 
absences  might  just  as  well  stand  for  actual  factors 
different  from  those  represented  by  the  large  letters. 
The  formulae  would  then  of  course  work  out  as  well 
as  before.  To  those  accustomed  to  the  presence  and 


THE    FACTORIAL    HYPOTHESIS  219 

absence  scheme  it  may,  however,  be  difficult  to  think 
of  the  small  letters  as  anything  but  absences.  It 
may,  therefore,  be  helpful  to  represent  the  same 
formulae  with  other  letters. 

If  the  original  comb  was  single,  which  in  fact  is 
the  type  of  comb  of  the  wild  bird  from  which  the 
domesticated  races  have  come,  a  dominant  muta- 
tion from  A  to  A'  gave  rise  to  a  rose  comb;  another 
dominant  mutation  from  the  wild  type  that  changed 
B  to  B'  gave  rise  to  a  pea  comb;  a  third  but  recessive 
mutation  that  changed  C  to  C'  gave  rise  to  a  "comb- 
less"  comb.  The  normal  allelomorphs  would  be 
represented  by  the  same  letters  without  the  primes. 
The  formulae  (in  simplex)  for  the  combs  would  then 
be  as  follows: 

Wild  type  (single)  ABC 
Rose  A'B  C 

Pea  A  B'C 

Combless  ABC' 

The  walnut  comb  that  appears  when  pea  is  bred 
to  rose  is,  of  course,  the  double  dominant  form  A 'B'C. 

If  it  seems  desirable  to  use  letters  that  give  a  clue 
to  the  name  of  the  factor  for  which  they  stand,  either 
of  the  next  alternatives  would  cover  the  case  under 
discussion.  In  the  second  of  these  the  small  letters 
are  not  absences,  but  only  the  recessive  allelomorphs. 

Wild  type  (single)  P  R  C   or  p'r'C 

Rose  PR'C    orp'R'C 

Pea  P'RC   orPVC 

Combless  P  R  C'  or  pVc 


220          THE  FACTORIAL  HYPOTHESIS 

It  is  a  matter  of  little  theoretical  importance  what 
system  of  symbols  is  adopted,  unless  that  system 
proves  to  be  impracticable,  or  unless  it  implies  re- 
lations that  are  unnecessary  or  unjustifiable.  (See 
Appendix.) 

We  do  not  wish  to  appear  to  base  our  objection  to 
the  presence  and  absence  hypothesis  on  the  im- 
practicability of  its  nomenclature  in  a  new  field, 
but  rather  on  the  grounds  that  the  conception  of 
presence  and  absence  assumes  that  we  do  know 
something  about  the  relation  between  character 
and  factor  that  we  can  not  possibly  know.  To  as- 
sume the  absence  of  a  factor  from  the  absence  of  a 
character  is,  in  a  sense,  as  naive  as  it  was  to  assume 
that  an  animal  moved  toward  light  because  it  liked 
the  light. 

It  need  not  be  denied  that  losses  of  factors  may 
occur,  and  it  may  even  be  probable  that  a  loss  in  the 
germ  plasm  might  lead  to  a  loss  in  some  part  or  parts 
of  the  body,  but  there  still  remains  no  justification 
for  the  assumption  in  any  given  case  that  we  can 
infer  from  the  lack  of  a  character  in  an  animal  or 
plant  a  loss  of  factors.  Such  an  assumption  is  en- 
tirely gratuitous;  and  gives  a  totally  false  impression 
concerning  the  factorial  hypothesis  of  Mendelian 
heredity.  Moreover,  if  taken  literally  it  may  lead 
to  unwarranted  conclusions  in  other  fields. 

It  is  similarly  naive  to  assume  the  absence  of  a 
factor  from  the  recessiveness  of  the  character,  yet 
the  literature  abounds  with  instances  where  the  re- 
cessiveness of  the  character  is  taken  as  a  criterion  for 


THE  FACTORIAL  HYPOTHESIS         221 

assuming  the  absence  of  the  factor,  the  dominant 
character  being  considered  as  a  "presence."  Domi- 
nance, however,  is  often  found  to  be  incomplete  if 
exact  quantitative  studies  are  made.  In  fact,  char- 
acters are  known  to  show  all  degrees  of  dominance 
and  recessiveness  over  their  alternative  allelomorphs. 
Which  character  is  to  be  considered  dominant  and 
which  recessive  when  each  allelomorph  has  an  equal 
effect,  as  in  the  case  of  the  red  and  the  white  Mira- 
bilis,  is  entirely  a  matter  of  choice.  Hence,  no  matter 
whether  red  or  white  is  presence,  the  present  factor  is 
not  truly  dominant.  It  seems  reasonable,  then,  to 
suppose  that  if  presence  and  absence  is  true  a  hybrid 
(with  one  presence)  might  approach  more  nearly  the 
type  with  two  absences  than  to  the  type  with  two 
presences.  In  such  a  case  the  present  factor  would 
actually  be  the  recessive.  Such  a  case  is  in  fact 
known.  In  the  cross  of  horned  by  hornless  sheep, 
the  horned  condition  dominates  in  one  sex  and  the 
hornless  in  the  other.  Here  no  matter  which  is 
considered  as  a  presence  it  must  be  conceded  that  in 
one  sex  or  the  other  it  is  recessive.  The  view  that 
dominance  of  a  factor  proves  its  presence  and 
recessiveness  its  absence  should  therefore  be  aban- 
doned. 

A  further  argument  against  the  theory  of  presence 
and  absence  is  found  in  the  evidence,  already  given, 
which  indicates  the  possibility  of  multiple  allelo- 
morphs. On  the  presence  and  absence  system,  only 
two  kinds  of  allelomorphs,  the  presence  and  the 
absence,  are  possible,  and  no  character  differences 


222  THE    FACTORIAL    HYPOTHESIS 

can  be  due  to  different  kinds  of  factors,  all  of  them 
"  presences." 

A  word  here  may  not  be  out  of  place  concerning 
inhibitors.  As  pointed  out,  the  adherents  of  presence 
and  absence  generally  interpret  the  absence  of  a  char- 
acter to  mean  the  absence  of  a  factor;  they  also  inter- 
pret recessiveness  to  mean  the  absence  of  a  factor. 
When  cases  come  up  in  which  a  character  is  absent, 
as  horns  in  cattle,  but  the  absence  of  the  character 
is  dominant,  an  attempt  is  made  to  reconcile  fact  and 
theory  by  assuming  that  the  factor  for  the  absent 
character  is  not  really  absent,  but  that  an  inhibitor 
is  present  whose  activity  prevents  the  appearance  of 
the  character. 

Those  who  do  not  accept  the  presence  and  ab- 
sence hypothesis  need  make  no  such  assumption  here 
of  course.  To  them  there  is  no  reason  why  a  factor 
for  hornless  should  not  dominate  a  factor  for  horns. 
Moreover,  the  facts  do  not  even  require  one  to  assume 
that  the  hornless  race  differs  from  the  horned  because 
of  the  lack  or  inhibition  of  certain  reactions,  for  it  is 
possible  in  such  cases  that  the  reaction  merely  takes 
a  different  course,  or  may  even  proceed  beyond  the 
usual  point. 

These  statements  are  not,  however,  intended  to 
mean  that  factors  may  not  at  times  act  as  inhibitors, 
but  rather  that  we  do  not  know,  and  in  most  cases 
can  not  know,  in  a  single  case  enough  about  the 
nature  of  the  reaction  to  demonstrate  the  existence 
of  a  factorial  inhibitor. 


THE  FACTORIAL  HYPOTHESIS  223 

WEISMANN'S  PR^EFORMATION  HYPOTHESIS  AND  THE 
FACTORIAL  THEORY 

Weismann's  theory  of  development  postulates  par- 
ticles in  the  germ  plasm  that  are  sorted  out  in  proper 
sequence  to  appropriate  parts  of  the  body  as  the 
embryonic  cells  divide.  What  determines  the  order 
of  the  sorting  out  of  the  factors  was  not  explained. 
Weismann's  speculation  differed  from  other  prseform- 
ation  theories  mainly  in  that  he  made  use  of  the 
chromosomal  mechanism  not  only  to  carry  the  here- 
ditary materials,  but  also  to  bring  about  the  sorting 
out  of  the  materials  in  order  to  reach  their  final  desti- 
nation in  the  body.  His  theory  as  applied  to 
embryonic  development  failed,  both  because  the  facts 
concerning  the  behavior  of  the  chromosomes  during 
segmentation  of  the  egg  gave  no  support  to  his  as- 
sumption of  sorting  out  of  the  materials  of  the  chromo- 
somes, and  also  because  the  data  from  experimental 
embryology  and  regeneration  indicated  very  clearly 
that  no  such  sorting  process  takes  place.  On  the 
other  hand,  Weismann's  ideas  of  heredity  concern- 
ing the  segregation  in  the  reduction  divisions  of  the 
egg  and  sperm  of  inherited  materials  present  in  the 
chromosomes,  furnish  the  basis  of  our  present  at- 
tempt to  explain  heredity  in  terms  of  the  cell. 

In  common  with  Weismann's  theory,  the  factorial 
theory  of  heredity  rests  on  the  assumption  that  the 
germ  plasm  contains  a  host  of  elements,  that  are  in- 
dependent of  each  other  in  the  sense  that  one  allelo- 
morph may  be  substituted  for  another  one  without 


224         THE  FACTORIAL  HYPOTHESIS 

alteration  of  either,  and  that  these  allelomorphs  will 
now  perpetuate  themselves  unchanged  although  in 
company  with  different  factors.  Today  this  as- 
sumption is  no  longer  an  a  priori  deduction,  but  a 
conclusion  from  experimental  data. 

The  second  real  and  important  point  of  agreement 
between  the  factorial  theory  and  Weismann's  theory 
is  that  both  maintain  that  at  one  period  in  the  history 
of  the  germ  cells,  the  factors  derived  from  the  mother 
separate  from  those  derived  from  the  father,  each 
pair  by  itself.  The  precise  way  in  which  this  is 
supposed  to  take  place  may  differ  slightly  on  the 
two  views,  but  the  essential  point  is  the  same.  We 
owe  to  Weismann  more  than  to  any  other  biologist 
the  conception  of  segregation  at  the  reduction  di- 
vision of  the  egg  and  sperm — a  conception  of  funda- 
mental importance  in  the  application  of  the  chromo- 
some theory  to  Mendelian  heredity.  The  factorial 
hypothesis  postulates  only  three  things  about  the 
factors  with  which  it  works,  viz.:  (1)  that  they  are 
constant,  (2)  that  they  are  usually  in  duplicate  in 
each  cell  of  the  body,  and  (3)  that  they  usually  segre- 
gate in  the  maturing  germ  cells.  But  the  biologist 
is  not  likely  to  stop  here,  for,  to  him  the  problem  in- 
volves cells  about  whose  history  and  processes  he  has 
come  to  know  certain  facts.  Weismann,  following 
Roux,  was  the  first  to  point  out  that  these  facts  give  a 
mechanism  showing  how  separation  of  factors  might 
take  place.  The  specific  application  of  the  behavior 
of  the  chromosomes  to  heredity,  then,  is  the  third 
important  contribution  which  modern  genetics  owes 


THE    FACTORIAL    HYPOTHESIS  225 

to  Weismann.  Today,  however,  we  have  advanced 
beyond  Weismann  in  this  respect,  and  may  more 
specifically  interpret  our  numerical  results  of  inde- 
pendent segregation,  linkage,  and  even  crossing  over 
on  the  basis  of  a  chromosome  mechanism.  More- 
over, the  new  facts  have  given  us  ideas  very  different 
from  those  of  Weismann  regarding  the  arrangement 
of  the  factors  in  the  chromosomes  and  the  way  in 
which  the  characters  of  an  individual  are  determined 
by  the  chromosomal  factors. 

In  the  last  edition  of  his  Vortrsege  ueber  Descen- 
denztheorie  (3d  edition,  1913)  Weismann  modifies 
his  earlier  views  in  regard  to  the  factorial  nature  of 
the  chromosomes  so  that  his  conception  of  the  germ 
plasm  is  brought  into  harmony  with  the  Mendelian 
theory  of  heredity.  Formerly  he  had  supposed  that 
the  chromosomes  are  all  alike,  or  nearly  alike,  in  so 
far  as  each  one  carries  a  full  assortment  of  "ids." 
Each  id,  in  itself,  represented  the  full  complement  of 
all  the  factors  that  go  to  make  up  the  organism. 
But  since  the  results  of  Mendelian  heredity  show  that 
all  sorts  of  characters,  however  trivial,  may  be  segre- 
gated independently  (which  would  not  be  the  case, 
if,  as  Weismann  formerly  supposed,  all  the  heredi- 
tary characters  are  carried  by  each  chromosome), 
it  follows  that  the  chromosomes  must  be  bearers  of 
part  ids  (Theil  Ids). 

Weismann  still  adheres  nevertheless  to  his  mosaic 
theory  of  development,  but  as  before  stated  the 
modern  work  on  development  does  not  support  this 
interpretation  of  development.  His  view  assumes 


226         THE  FACTORIAL  HYPOTHESIS 

disintegration  of  the  germ  plasm  when  the  body  cells 
are  produced  in  order  to  account  for  the  localization 
of  characters;  the  other  view,  following  the  experi- 
mental results  and  microscopical  observations,  as- 
sumes, so  far  as  the  chromosomal  materials  are  con- 
cerned, that  all  of  the  hereditary  factors  are  present 
in  every  cell  in  the  body.  This  view  is  essentially 
that  proposed  by  DeVries  in  his  book  on  Intracellular 
Pangenesis.  The  cause  of  the  differentiation  of  the 
cells  of  the  embryo  is  not  explained  on  the  factorial 
hypothesis  of  heredity.  On  the  factorial  hypothesis 
the  factors  are  conceived  as  chemical  materials  in  the 
egg,  which,  like  all  chemical  bodies,  have  definite 
composition.  The  characters  of  the  organism  are 
far  removed,  in  all  likelihood,  from  these  materials. 
Between  the  two  lies  the  whole  world  of  embryonic 
development  in  which  many  and  varied  reactions 
take  place  before  the  end  result,  the  character,  emerges. 
Obviously,  however,  if  every  cell  in  the  body  of  one 
individual  has  one  complex,  and  every  cell  in  the 
body  of  another  individual  has  another  complex  that 
differs  from  the  former  by  one  difference,  we  can  treat 
the  two  systems  as  two  complexes  quite  irrespective 
of  what  development  does  so  long  as  development  is 
orderly. 

It  is  sometimes  said  that  our  theories  of  heredity 
must  remain  superficial  until  we  know  something 
of  the  reactions  that  transform  the  egg  into  the  adult. 
There  can  be  no  question  of  the  paramount  impor- 
tance of  finding  out  what  takes  place  during  develop- 
ment. The  efforts  of  all  students  of  experimental 


THE    FACTORIAL    HYPOTHESIS  227 

embryology  have  been  directed  for  several  years 
toward  this  goal.  It  may  even  be  true  that  this 
information,  when  gained,  may  help  us  to  a  better 
understanding  of  the  factorial  theory — we  can  not 
tell;  for  a  knowledge  of  the  chemistry  of  all  of  the  pig- 
ments in  an  animal  or  plant  might  still  be  very  far 
removed  from  an  understanding  of  the  chemical 
constitution  of  the  hereditary  factors  by  whose 
activity  the  pigments  are  ultimately  produced. 
However  this  may  be,  the  far-reaching  significance 
of  Mendel's  principles  remains,  and  gives  us  a 
numerical  basis  for  the  study  of  heredity.  Although 
Mendel's  law  does  not  explain  the  phenomena  of 
development,  and  does  not  pretend  to  explain  them, 
it  stands  as  a  scientific  explanation  of  heredity, 
because  it  fulfils  all  the  requirements  of  any  causal 
explanation. 


APPENDIX 

METHODS   OF  BREEDING   DROSOPHILA 

Drosophila  ampelophila  has  shown  itself  to  be  so 
generally  useful  for  class  work  in  genetics  and  is 
being  so  widely  employed  for  this  purpose  that  it 
may  not  be  out  of  place  here  to  give  a  few  directions 
concerning  apparatus,  methods  and  material. 

CULTURE  BOTTLES. — Large-mouthed  bottles  of 
about  500  c.c.  capacity  should  be  used.  Pint  milk 
bottles  can  be  purchased  at  reasonable  rates  from 
wholesale  dealers,  and  serve  admirably  as  culture 
bottles.  Stoppers  of  raw  cotton  are  used,  which 
should  be  tight  but  not  packed  in  the  mouth  of  the 
bottle. 

TEMPERATURE. — The  optimum  is  about  25°  C. 
Extreme  summer  heat  kills  the  flies  in  culture  bottles 
unless  special  precautions  are  taken.  Cold  retards 
the  development  of  the  larvaB  indefinitely,  but  the 
flies  themselves  can  withstand  almost  a  freezing 
temperature.  Ordinary  room  temperature  suffices, 
as  a  rule,  but  a  controlled  temperature  of  about 
25°  C.  is  better. 

FOOD. — Ripe  fermented  banana  is  the  best  food. 
If  raw  bananas  with  intact  skin  are  peeled  and  put 
into  old  juice  contamination  is  not  likely  to  occur, 
but  for  ordinary  purposes  the  bananas  should  be 

229 


230  APPENDIX 

peeled,  broken  up  into  pieces,  covered  with  water, 
and  slowly  brought  to  nearly  the  boiling  point,  or 
else  steamed.  The  pieces  are  then  put  into  the  old 
juice.  This  juice  is  made  in  the  first  instance  by 
adding  a  little  yeast  to  the  water  that  covers  the 
bananas  (after  they  have  cooled).  The  juice  can 
then  be  used  over  and  over  again  if  it  is  occasionally 
greatly  diluted  with  water  to  keep  it  from  becoming 
too  acid.  The  food  is  at  its  best  from  one  to  two 
days  after  being  in  the  juice,  although  it  may  be  used 
for  a  week  or  more.  Keep  food  in  glass-stoppered 
large-mouthed  jars.  Scrupulously  avoid  leaving  the 
margin  of  the  jar  wet  after  removing  food. 

FEEDING. — When  a  culture  is  to  be  made  up  the 
most  approved  way  is  to  put  about  a  teaspoonful 
of  food  on  the  bottom  of  the  bottle  and  over  it  a 
folded  piece  of  absorbent  paper.  The  flies,  while 
still  under  ether,  may  be  dropped  into  a  cornucopia 
of  paper  which  is  placed  in  the  bottle.  Often,  how- 
ever, the  flies  are  brushed  into  a  dry  bottle  and  the 
cotton  plug  put  in.  An  hour  or  more  later,  when 
the  flies  have  recovered,  the  food,  wrapped  up  in 
paper,  is  added.  Toweling  paper  is  cheap  and  serves 
excellently  the  purpose  of  wrapping  the  food,  etc. 

By  putting  in  at  first  more  food  than  recommended 
above  it  is  possible  to  carry  a  culture  to  the  end  with- 
out further  feeding.  But  more  flies  and  greater 
accuracy  result  if  a  small  amount  of  food  is  first  added, 
then  as  much  more  at  the  end  of  six  or  seven  days. 
When  the  parent  flies  are  taken  out  at  about  the  tenth 
day,  food  may  be  added  for  a  third  time.  New 


APPENDIX  231 

food  should  not  be  allowed  to  cover  the  old  food  and 
thereby  drown  the  pupae. 

PRECAUTIONS. — In  most  cases  a  single  female 
with  one,  or  in  some  cases  more  males  should  be 
put  into  a  bottle,  except  for  stock  breeding,  when 
more  flies  should  usually  be  used.  Under  favorable 
conditions  200  to  300  flies  should  be  obtained  within 
the  first  ten  days  after  hatching  has  commenced. 
The  parents  should  be  removed  about  ten  days  after 
the  beginning  to  avoid  overlapping  of  generations. 
If  desirable  they  may  then  be  transferred  to  a  second, 
or  even  to  a  third  culture  bottle.  In  summer 
the  parents  should  not  remain  over  eight  days  in 
the  old  bottle;  in  winter  they  may  be  left  more  than 
ten  days  with  safety.  To  obtain  virgin  females  for 
mating,  the  bottles  should  be  thoroughly  emptied — 
it  may  be  necessary  to  remove  the  old  paper,  etc., 
in  order  to  make  certain  that  all  of  the  old  flies  are 
removed — and  females  obtained  not  later  than  six 
hours  after  the  bottle  was  emptied.  Females  ob- 
tained twelve  hours  after  the  bottle  was  emptied  are 
not  certainly  virgin.  If  the  old  males  have  not  been 
removed  females  so  young  as  to  have  their  wings  not 
yet  unfolded  may  in  rare  cases  have  already  under- 
gone copulation. 

EXAMINATION. — The  flies  go  toward  the  light. 
Therefore,  if  the  bottle  is  held  with  its  mouth  away 
from  the  light,  the  flies  are  not  likely  to  escape. 
The  plug  may  then  be  removed  and  another  smaller 
bottle,  with  a  mouth  that  fits  the  larger  one,  placed 
mouth  to  mouth  against  the  larger  bottle.  The  com- 


232  APPENDIX 

bination  is  then  turned  around,  and  the  flies  fly  into 
the  smaller  bottle — or  the  smaller  bottle  may  be  held 
firmly  underneath  the  other  and  the  flies  shaken  into 
it  by  jarring.  The  small  bottle  can  then  be  plugged, 
and  a  bit  of  cotton  with  four  or  five  drops  of  ether 
put  into  it.  In  a  minute  or  two  the  flies  are  under 
the  influence  of  the  ether  and  may  be  emptied  out 
on  to  a  piece  of  white  paper,  or  a  white  glass  plate. 
Some  workers  prefer  to  have  the  small  bottle  already 
saturated  with  ether  before  the  flies  are  shaken  into 
it;  in  this  case  they  become  etherized  almost  imme- 
diately. They  can  then  be  examined  with  a  hand 
lens  or  with  a  binocular  microscope.  Some  of  the 
characters  require  for  study  the  latter  or  an  ordinary 
microscope. 

With  a  camel's  hair  brush  the  flies  are  pushed  out 
into  a  row  and  then  sorted  out,  from  right  to  left, 
into  an  upper  and  a  lower  row,  each  of  which  may 
again  be  subdivided.  If  overetherized,  the  wings 
stand  out  above  and  at  right  angles  to  the  body.  If 
insufficiently  etherized,  so  that  they  recover  before 
they  can  be  examined,  they  may  be  etherized  again. 

The  pure  stock  is  kept  in  pint  bottles  and  new 
cultures  made  up  each  week.  Descriptions  of  the 
mutants  as  far  as  published  will  be  found  in  the  fol- 
lowing journals: 

BAR. — "A  New  Sex-linked  Character  in  Drosophila."    Biol.  Bull., 

XXVI.     1914. 
BEADED. — "The  Analysis  of  a  Case  of  Continuous  Variation,  Etc." 

Amer.  Nat.,  XLIII.     1914. 
BENT. — "A  Gene  for   the    Fourth  Chromosome   of    Drosophila." 

Jour.  Exper.  Zoo/.,  XVII.     1914. 


APPENDIX  233 

BLACK. — "Heredity  of  Body  Color  in   Drosophila."     Jour.  Exper 

Zool.,  XIII.     1912. 
CHERRY.— "A  New  Eye  Color  Mutation  in  Drosophila,  Etc."     Biol. 

Bull.,  XXV.     1913. 
EBONY. — "A  Third  Group  of  Linked  Genes  in  Drosophila."     Science 

XXXVII.     1913. 
EOSIN. — "Dilution   Effects    and    Bicolorism,  Etc."     Jour.    Exper. 

Zool.,  XV.     1913. 

EYELESS. — "Another  Gene  in  the  Fourth  Chromosome  of  Droso- 
phila."    Amer.  Nat.,  XLIX.     1915. 
LETHAL. — "Two  Sex-linked  Lethal   Factors   in    Drosophila,    Etc." 

Jour.  Exper.  Zool.,  XVII.     1914. 
MINIATURE. — "A   Modification   of  the  Sex  Ratio,  Etc."     Zeits.  f. 

ind.  Abst.-  u.  Vererb.-Lehre.,  VII.     1912. 
PINK. — "Dilution    Effects    and    Bicolorism,    Etc."     Jour.    Exper. 

Zool.,  XV.     1913. 
RUDIMENTARY. — "A  Modification  of  the  Sex  Ratio,  Etc."     Zeits.  f 

ind.  Abst.-  u.  Vererb.-Lehre.,  VII.     1912. 
SPOT. — "Another  Case  of   Multiple  Allelomorphs  in   Drosophila." 

Biol.  Bull,  XXVI.     1914. 
VERMILION. — "Dilution. Effects  and  Bicolorism,  Etc."     Jour.  Exper. 

Zool.,  XV,  1913. 
VESTIGIAL. — "No  Crossing  Over  in  the  Male  of  Drosophila,  etc." 

Biol.  Bull.,  XXVI.     1914. 
WHITE. — "Sex     Limited      Inheritance    in     Drosophila."     Science, 

XXXII.    1910. 
YELLOW. — "Heredity  of  Body  Color  in  Drosophila."     Jour.  Exper. 

Zool.,  XIII.     1912. 

FORMULAE 

Baur's  plan  of  using  non-significant  letters  has 
no  doubt  certain  advantages,  but  in  practice  signifi- 
cant letters  are  too  useful  to  be  given  up.  We  have 
followed  a  plan  which  avoids  the  objections  of  the 
presence  and  absence  scheme,  and  has  the  advantage 
of  significant  letters.  In  this  plan  a  small  letter  is 
used  for  the  mutant  factor  if  recessive,  and  a  large 


234  APPENDIX 

letter  if  dominant.  With  a  little  practice  we  have 
found,  from  our  own  experience,  there  is  no  real 
difficulty  in  making  the  transition  from  the  presence 
and  absence  notation  to  this  one.  For  example : 

Simplex     Duplex 
Pinkeye 


PvW 

Vermilion  eye    PvW  — >  w~Tir 

F  V  W 

PVw 
White  eye          PVw  ^  PVw 

PVW 

Red  eye  PVW  -*  p^ 


A  further  simplification  would  consist  in  using  the 
letters  for  the  mutant  factors  alone,  as  Castle  has 
done,  and  omitting  the  normal  factors.  But  in 
writing  out  formulae  for  heterozygous  forms,  it  is 
often  convenient  to  represent  both  members  of  a 
pair  of  allelomorphs.  In  matters  relating  to  link- 
age it  is  essential  to  indicate,  in  some  way,  both 
allelomorphs. 

If  in  any  formula  it  is  desirable  to  distinguish  be- 
tween dominant  and  recessive  mutant  factors,  it  may 
be  convenient  to  prime  both  allelomorphs  of  a  pair 
in  which  the  factor  is  named  from  the  dominant 
character. 

In  addition  to  the  more  important  objections  to 
the  presence  and  absence  representation  that  have 
been  dealt  with  in  the  text,  there  are  certain  tech- 
nical drawbacks  to  the  presence  and  absence  scheme 
of  nomenclature  that  should  not  pass  unnoticed. 


APPENDIX  235 

When  to  a  familiar  or  to  an  established  system  it 
becomes  necessary  to  add  new  recessive  types  diffi- 
culties arise.  This  may  be  illustrated  in  the  case 
of  combs  of  fowls,  the  main  facts  concerning  whose 
inheritance  have  been  discussed  on  page  216.  On 
the  presence  and  absence  scheme  a  factor  gets  its 
name  from  the  effect  that  that  factor  produces  in  the 
absence  of  other  factors  affecting  the  character.  The 
factor  for  pea,  for  instance,  got  its  name  from  the 
effect  produced  when  a  factor  for  rose  was  supposed 
to  be  absent;  and  the  formula  for  single  comb,  rpS, 
means  that  a  f  ac  tor  S,  for  single,  produced  its  particular 
effect  when  the  two  other  factors  were  absent.  When 
a  new  condition,  combless,  was  met  with  and  added  to 
the  series  it  was  represented  as  due  to  the  loss  of  the 
factor  (S)  for  single.  The  formula  for  combless 
became  rpsB  (B  standing  for  the  vestige  of  a  comb, 
called  Breda,  that  remained).  What  now  is  the 
factor  for  rose  comb?  Originally  this  factor  got  its 
name  from  its  effect  in  the  absence  of  pea  but  in  the 
presence  of  S  (RpS) ;  now  the  factor  for  rose,  R,  should 
be  re-named  from  its  effect  in  the  absence  of  both 
factors  P  and  S.  The  series  must  then  be  re-con- 
structed on  a  new  basis  and  the  same  process  must 
be  gone  through  with  whenever  a  new  factor  is 
brought  into  relation  with  an  established  system. 

ACKNOWLEDGMENTS 

We  wish  to  express  our  indebtedness  to  Miss  E. 
M.  Wallace  for  her  skill  in  making  many  of  the  illus- 
trations, and  also  to  Miss  M.  L.  Hedge  who  has 


236  APPENDIX 

helped  us  likewise.  The  text  has  been  gone  over 
in  parts  by  Dr.  F.  N.  Duncan,  Mr.  Alexander 
Weinstein,  Mr.  E.  Altenburg  and  Mr.  D.  B.  Young; 
we  wish  to  express  our  appreciation  for  the  help 
they  have  given.  Four  well-known  geneticists  have 
looked  through  the  last  three  chapters  and  have 
made  valuable  suggestions.  They  are  not  named 
here  lest  we  appear  to  commit  them  to  opinions  with 
which  they  may  not  agree  in  all  details,  but  on  the 
whole  we  know  that  they  do  in  general  agree  with 
the  interpretation  of  the  factorial  hypothesis  that 
we  have  followed.  We  express  to  them  individually 
our  appreciation  of  their  advice  and  criticism. 


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WOLTERECK,    R.,    1911.     Uber    Veranderung    der    Sexualitat    bei 

Daphniden.    Leipzig,  1911. 
WOODRUFF,  L.  L.,  1905.     An  Experimental  Study  of  the  Life  History 

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WOODRUFF,  L.  L.,   1908.     The  Life  Cycle  of  Paramsecium  when 

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INDEX 


INDEX 


Abnormal  abdomen,  39-41,  47 

Abnormal  venation,  195-196 

Abraxas,  70,  78-79,  83-88,  153-154 

Age,  42 

Altenburg,  191 

Amitosis,  118 

Andalusian  fowl,  28-29 

Antirrhinum,  69,  138 

Aphids,  97 

Arc  wings,  37 

Ascaris,  79,  119,  129-131 


B 


Back  cross,  50 

Baltzer,  96-97,  114-117,  121 

Band,  206 

Bar  eye,  29-30,  58-59,  62-64,  232 

Bateson,  5,  74-76,  173,  214-216 

Batracoseps,  123-125,  132 

Baur,  38,  138-139,  157,  233 

Beaded  wing,  194-195,  232 

Beans,  159,  204-205,  213 

Belling,  190 

van  Beneden,  129 

Bierens  de  Haans,  110 

Bifid  wings,  64 

Biston,  144-145 

Black  color,  37,  42,  48-53,  181-182, 

233 

Bonellia,  96-97 
Boveri,  110-113,  119 
Bow  wings,  37 
Brauer,  129 


Bryonia,  79 
Bursa,  175-178 


C 


Canaries,  70,  79 

Carothers,  140 

Castle,  187-188,  190,  196-202,  205 

Cat,  70,  79,  83 

Cattle,  222 

Cherry  eyes,  164-166,  233 

Chiasmatype,  132-134 

Chickens  (see  Fowls). 

Chlorophyll  grains,  137-139 

Chromosomes,  2,  7-8 

Club,  32-34 

Color  blindness,  83 

Columbine,  157 

Corn,  26,  161-164,  179,  188-190 

Correns,  137-138 

Cotton,  26 

Coupling,  5 

Cream  color,  203 

Crossing  over,  48-71,  131-135 

Ctenolabrus,  121 

Cudnot,  159,  190 

Cumulative  factors,  174 

D 

Daphnians,  97 
Davenport,  190,  212 
Dederer,  90 
Dexter,  194-195 
Differentiation,  43-45 
Dominance,  31-32 


259 


260 


INDEX 


Doncaster,  83-88,  90,  121,  144-145, 
153-154 

Driesch,  113 

Drosophila,  5-26,  29-35,  37-43, 
45-70,  78-83,  90-92,  105, 
132,  134,  148-153,  154- 
157,  164-167,  173-174, 
181-182,  191-196,  203, 
205-209,  229-234 

Ducks,  79 

Dunaliella,  97 

E 

East,  179,  183-185,  189-190 
Ebony  color,    13,  21-25,  37,    167, 

181-182,  233 

Emerson,  159,  161,  164,  170,  190 
Environment,  38-42 
Eosin  eyes,  155-156,  165-166,  233 
Evening  primrose  (see  CEnothera). 
Eye  color,  208-209 
Eyeless,  13-14,  25,  233 


F 


Facet,  38 

Factors,  3 

Fantails,  186-187 

Faust,  106 

Federley,  121,  141-144 

Formulae,  233-235 

Four  o'clock,  26,  27-28,  137-138 

Fowls,    28-29,   36,    70-73,  79,  90, 

179,  216-219 
Fractionation,  214-216 
Fringed  wings,  37 
Fundulus,  121 

G 

Gates,  146 
Gregory,  147-148 


H 

Haemophilia,  83 

Hagedoorn,  190 

Hayes,  190 

Helix,  26 

Herbst,  110,  116-117,  121 

Heribert-Nilsson,  190 

Hertwig,  G.,  121 

Hertwig,  O.,  129 

Hertwig,  P.,  121 

Hipponoe,  116 

Hoge,  41 

Holmes,  212 

Hurst,  212 

Hybrids,  120-121,  141-146 

Hydatina,  97-99 


Ids,  225 
Inhibitors,  222 
Interference,  64 


Janssens,  123-125,  132-134 
Jaunty  wings,  37 
Johannsen,  190,  204-205,  213 


K 


Kajanus,  190 


Leptinotarsa,  26,  43-44 

Lethal  factor,  105,  233 

Linkage,  5 

Lock,  5 

Lutz,  A.  M.,146 

Lutz,  F.  E.,  195-196 


INDEX 


261 


Lychnis,  7,  79,  138,  159 
Lymantria,  70 

M 

MacDowell,  185,  187,  190 

Man,  26,  79,  83,  212 

Marechal,  125-127 

Maturation,  59-60,  122-131 

Melandrium  (see  Lychnis). 

Mendel,  1,  30,  227 

Menidia,  121 

Metapodius,  120,  140 

Miniature  wings,  30,  58,  62-63,  233 

Mirabilis  (see  Four  o'clock). 

Moenkhaus,  120 

Morris,  121 

Mouse,  26,  45,  159,  161,  179-180, 

190 
Mutations,  35,  205-207 


N 


Nabour,  162-163 

Nicotiansa,  26,  183-185,  190    (see 

Tobacco). 
Nightshade,  26 

Nilsson-Ehle,  174,  178-179,  190 
Non-disjunction,  7,  80,  149-154 

O 

Oats,  179,  190 
(Enothera,  26,  146-147 


Paratettix,  162-163 
Pea,  1,  26,  69 
Peach  eyes,  166 
Pearl,  190 
Pelargonium,  138-139 


Phillips,  190,  196-197,  201 

Phragmatobia,  88-90 

Phylloxera,  99-105 

Pigeons,  70,  186-187 

Pink  eyes,  37,  42,  45,  47,  166,  173, 

233 

Potato  beetle  (see  Leptinotarsa). 
Prseformation,  223 
Presence  and  absence,  216-222 
Primrose  (see  Primula). 
Primula,  38,  69,  147-148 
Pristiurus,  125-127 
Punnett,  5,  76 
Pure  line,  213 
Purple  eyes,  42,  45 
Pygsera,  141-144 


R 


Rabbits,  157-158,  185-188,  190 
Rats,  196-202 
Reduplicated  legs,  41-42 
Reduplication,  74-77 
Rosenberg,  121 
Rough,  38 
Roux,  77,  224 
Ruby  eyes,  37 

Rudimentary    wings,    34-35,    38, 
209,  233 


Sable  color,  37,  63-64 

Sea  urchins,  110-118 

Segregation,  3 

Seiler,  86,  88-90 

Sex  chromosomes,  14-16,  78-90 

Sex  limited,  94-95 

Shull.  A.  F.,  97 

Shull,  G.  H.,  159,  175-178,  190 

Silkworm,  37,  69,  136-137,  159-160 

Snapdragon  (see  Antirrhinum). 


262 


INDEX 


Sooty  color,  166-167 

Sphserechinus,  114-116 

Spiders,  79 

Spot,  167,  233 

Spread  wings,  37 

Stature,  173 

Stevens,  90 

Stocks,  69 

Streak,  207 

Strongylocentrotus,  114-116 

Sutton,  4 

Sweet  peas,  5,  36,  69,  174-175 


Tammes,  190 

Tanaka,  69,  159 

Tennent,  116,  121 

Tetrad,  128 

Thomas,  146 

Tobacco  (see  Xicotiana). 

Tomato,  26 

Tower,  43-44 

Toxopneustes,  116 

Trefoil,  206 

Trident,  205-207 

Trow,  75 

Truncate  wings,  34,  38,  191-194 

Tschermak,  190 


U 
Unit  character,  210 

V 

Vermilion  eyes,  45.  63-64,  233 
Vestigial  wings,  8-12,  21-25,  48-53, 

233 
de  Vries,  226 

W 

Weismann,  77,  211,  223-225 

Wheat,  26,  179,  190 

White  eyes,  16-20,  31,  45,  47,  54- 
59,  62-64,  82,  149-151, 
155- 156,  164-166,  208, 
233 

Whitney,  97 

Wilson,  120.  140 


Yellow  color,  54-58,  64,  156,  167, 
233 


Zeleny,  106 


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